Like at an intersection? I was thinking about this yesterday too. I am pretty sure it means like "hey this is an intersection and the other roads have a flashing red, please be alert but don't slow down unless necessary".

Whereas yellow flashing means "slow down and make sure it's safe before entering intersection, there might be cross-traffic" and red is "treat this like a stop sign"

Green flashing is for a pedestrian controlled intersection. It is special to indicate pedestrians are likely to cross the area, and the signal will change suddenly from a pedestrian hitting the signal to cross alewife Pkwy and Salem has examples.