The teacher tells the Blue team they do not have to capture the Red flag to win. Instead, they must keep the Red team from capturing all the Blue flags, just like
During the American Revolution the 13 US colonies (blue) were trying to get independence from Britain(red) they weren’t trying to get control of or conquer Britain(get their flag) instead they were trying to keep Britain from having complete control of them so they could free themselves from them( keep them from getting all their flags)