The Avatar State is a defense mechanism, designed to empower you with the skill and knowledge of all the past Avatars. The glow is a combination of all your past lives focusing their energy through your body. In the Avatar State, you are at your most powerful. But you are also at your most vulnerable. If you are killed in the Avatar State, the reincarnation cycle will be broken, and the Avatar will cease to exist.

(via daenarystormborn)