The Avatar State is an ability of the Avatar to call upon the powers of previous incarnations.
The Avatar State is an ability inherent to the Avatar that is called upon during dire emergencies in untrained Avatars or at will. The Avatar State allows the Avatar to use the powers of his or her previous lives in order to fight, even if they haven't learned that element yet. For example, Aang was able to enter the Avatar State and firebend even though he hadn't begun his training in it. The Avatar State can also be used to pass on techniques from previous lives, channel the spirit into a physical form, as well as communicate with past lives for guidance and answers. The only caution of using the Avatar State is that if an Avatar is killed while in the State, then the cycle will be broken and the Avatar will truly be dead.