The green dot on a football helmet signifies the player whose coach can talk to him through a radio. read more
The green dot on the back of a football helmet means that there is a small radio receiver and set of speakers inside that helmet so that the player wearing it can hear their coach talk to them at specific times during the game. read more
The helmets with the dots have a speaker in them so they can hear the coach call a play. The reason for the dot is that they only want 1 player on the field at 1 time to be able to hear. For example last year a team had 2 QB's on the field at the same time and they could both hear the coach. read more