More than 150,000 seniors in Eastern Massachusetts who are enrolled in traditional Medicare plans have received a letter in the mail - or will soon - informing them that their doctors are part of a newfangled health care system. Each accountable care organization, or ACO, is aimed at making sure patients get “the right care, in the right place, at the right time,’’ the letter says.

It’s likely that few, if any, know just what an ACO is. Until recently, many health policy experts were hard-pressed to explain exactly how the new health care model, formalized under the Affordable Care Act and launched by Medicare on Jan. 1, would work.

Medicare spent much of last year hammering out the details, but now 32 hospital and doctor groups, including five in Massachusetts, have signed on to be the first in the country to try the new model.

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The agency that oversees the federal health program for seniors will track how well the 32 ACOs do at lowering costs and improving care, and it will award bonus payments or assess penalties accordingly.

If the hospital and doctor groups cut the cost of treating their Medicare patients, they will get to keep a portion of what they save, with the rest going to the government. If they overspend, they could be stuck covering the extra costs themselves. Their financial gain or loss will depend in part on whether they improve the health of patients with certain common conditions tracked by Medicare.

While some experts say accurately tracking quality of care and cost will be a challenge, federal officials estimate that the program will save Medicare up to $1.1 billion over five years. But what could it mean for patients?

First, a note about what ACOs won’t do. The ACO model does not restrict which doctors or hospitals a patient can visit.

“This is a contract between the doctors and the people who are paying for health care,’’ said Dr. Tim Ferris, vice president for population health at Partners HealthCare, one of the five ACOs in Massachusetts. “Your benefits and your choice are in no way changed.’’

Partners, which includes Massachusetts General and Brigham and Women’s hospitals, has begun fielding calls from seniors looking for help in deciphering their letters, which also explain how seniors can opt out if they don’t consent to having their health information shared with the ACO. Some letter recipients, Ferris said, are worried that the change marks the return of the managed care model of the 1990s, when primary care doctors were seen as gatekeepers who controlled their bottom line by restricting care.

“It’s different this time,’’ he said.

For starters, patients who don’t like how they are being treated in an ACO can go elsewhere, an option not afforded under managed care. But more than that, the ACO is meant to reward doctors and hospitals for doing good work, said Dr. Richard Gilfillan, director of the Innovation Center that oversees the 32 Medicare Pioneer ACOs.

“I think we’ll see people making house calls, going into nursing homes, calling patients in more regularly, at times, to make sure that their health is improving,’’ he said. “People will feel a very different touch from the health care system as a result of this program.’’

Better access

Many people know the frustration of not having access to a doctor when they need it. What seems like a simple question about their health requires a trip to the office. But appointments are booked out for weeks. Getting one at night or on weekends is even harder.

Doctors traditionally get paid only for face-to-face encounters and specific tests or treatments. The ACO model aims to change that by shifting the payment system, rewarding doctors who keep their patients out of the hospital, even if that means serving them in new ways.

Atrius Health, an alliance of six community-based doctors groups that is part of the new ACO program but has long considered itself an accountable care organization, already has about 10 locations open on the weekends and is looking to expand its off-hours, said Dr. Rick Lopez, primary care internist and chief medical officer. Doctors also will have a greater incentive to communicate with patients by phone or by e-mail through the system’s secure web portal, when appropriate, he said.

“The motivation to do that right now is very slim because you don’t get paid to spend that 20 minutes on the phone with somebody,’’ said Dr. JudyAnn Bigby, state secretary of Health and Human Services, who last month announced an effort to move the hospitals in Massachusetts that serve the greatest number of poor patients into an accountable care model as well.

New faces

It can be hard for patients who have a chronic condition such as diabetes or heart failure to keep track of doctor appointments, medications, and necessary lifestyle changes. At least two-thirds of all Medicare enrollees have two or more chronic illnesses.

Primary care doctors and some specialists increasingly are leaning on other staff members to help care for such patients. Staff pharmacists explain patients’ medication schedule and watch for negative drug interactions, health coaches teach people about their diseases or help them to lose weight, and nurse care managers take on a wide variety of tasks, including making sure a patient gets necessary lab work and follow-up care.

Under the traditional health care model, Medicare won’t pay for most of the work they do. In ACOs, their role will grow, experts say.

Partners has nurse care managers in about half of its doctor offices now and plans to put them in all offices within two years, Ferris said. Atrius is expanding its ranks of care managers and clinical pharmacists, too.

“Patients will feel that there’s more contact, that there are people who know them better, who are looking after them and providing them with more feedback,’’ Gilfillan said.

More coordination

Medicare won’t judge the Pioneer ACOs only on the care they provide at their own facilities but on the care that their patients get throughout the health care system. That’s meant to encourage more coordination between hospitals and with other providers, so that physicians, for example, become more involved in the care their patients receive at rehabilitation facilities.

Three Massachusetts ACOs - Atrius, and the physician organizations affiliated with Mount Auburn Hospital and Beth Israel Deaconess Medical Center - are working together to evaluate the skilled nursing facilities they use. There is an overlap in the Medicare population that each sees, so it’s in their mutual interest to work together to make sure their patients are getting the best possible care, said Lopez of Atrius.

“It also gives us more leverage at these facilities to make sure that the care provided is what we need,’’ he said.

Atrius is also talking with the Mount Auburn group about hiring a case manager to work with Atrius patients who visit the Mount Auburn emergency department.

Steward Health Care, which operates 10 hospitals in Massachusetts, also is a Medicare ACO.

Data needed

Managed care organizations of the 1990s were set up to benefit everyone but patients, said Jeff Selberg, chief operating officer of the Cambridge-based Institute for Healthcare Improvement, which consults for some out-of-state ACOs.

“There was a real backlash,’’ he said.

This new program will only be successful if the ACOs truly do focus on improving patient care, Selberg said.

The Centers for Medicare and Medicaid Services has laid out 33 quality measures it will use to evaluate ACOs. Seven will be based on patients surveys, including questions about access to appointments and to specialists. The quality measures are a start, Selberg said, but not enough.

“We need to keep pressing for even more transparency,’’ he said. “There’s been progress. I don’t want to say there hasn’t been. But to say we’re satisfied with where we are? No way.’’

Try BostonGlobe.com today and get two weeks FREE. Chelsea Conaboy can be reached at cconaboy@boston.com. Follow her on Twitter @cconaboy.