From the introduction:
The Avatar, a central figure in the world of Avatar: the Last Airbender, is perceived to be the physical incarnation of the spirit of the world. The Avatar serves to maintain a balance of power between the four human nations, promote peace and prosperity throughout the world, protect the environment, and bridge the gap between the world of the spirits and the physical world.
The Avatar's unique ability to master all four of the bending disciplines and use them in combination makes the Avatar extremely powerful. In addition to the disciplined mastery of all the elements, the Avatar can, in times of great danger or emotional stress, enter into a special "Avatar State," wherein the Avatar can call on the knowledge and power of all the past Avatars. The Avatar's eyes glow and his or her power increases substantially. The Avatar State even allows the Avatar to use bending abilities he or she would normally be unable to perform. Finally, through intense meditation, the Avatar can enter into the spirit world and interact with spirits.