The Avatar State is an ability of the Avatar to call upon the powers of previous incarnations.
The avatar state is the most powerful ability given to the avatars. It was first created when the light spirit known as Raava fused with the first avatar, Wan. The ability grants the avatar an increase in their bending abilities and access to known from their previous incarnations. Once in this state, the avatar is allowed to use skills or bend elements that he or she would not be able to use normally. For example, Aang is able to bend the earth at the beginning of the second book even though he never learned how to bend earth. The avatar state also grants the avatar the ability to act as a medium for spirits.
For unrealized avatars, the avatar state can be triggered only by emotional distress or danger. When these avatars enter the avatar state, their eyes and any tattoo near their chakra paths glow. In this state, they lose their ability to control their actions and are controlled by the avatar spirit. Once an avatar has mastered the avatar state, they can go into the state and out of it at will. They also have full awareness of their actions while in this state.
While the avatar state does present great advantages, there is great disadvantage. If the avatar is killed while in the avatar state, the avatar cycle will end, meaning that avatar won't be reincarnated. Because of this, the avatar state is meant to be a defensive mechanism used when the avatr has no other option. Fully realized avatars enter this state briefly to take the energy they need to perform a specific task and leave it quickly to minimize this risk.