An estimated 400 people gathered this afternoon on the Summer Street bridge in the annual “Stand Against Racism” rally, with many saying a flap over racist tweets after Wednesday’s Bruins game highlighted the need to make more progress against bias. “I guess you don’t have to look any further than what happened on Wednesday. That’s all the more reason why activities like this are important,” said Charlayne Murrell-Smith, 61, of Roxbury.