What's with the green dot on QB helmets?

I was wondering this myself, but my mighty google-fu found the answer for me. As a service to our readers for whom the force is not as strong, I present the knowledge you seek (from here):

For anyone wondering why there is a green dot on the back of every quarterback’s helmet, it is to signal that they have been designated to wear a transmitter in their helmet to receive calls from the sidelines.

Had the NFL passed a rule to allow defenses to have one player use a transmitter, that player would have had a green dot on their helmet as well.