Energybending is the ability to bend life energy. It predates the four main bending arta, as well as the arrival of the Avata and the formation of the four nations.
Before the arrival of the Avatar into the world and the separation of the four elements, people bent the energy within themselves. At some point, they discovered the other four bending arts, and, over time, the knowledge to perform energybending was almost completely lost. The last known keeper of the art was the ancient lion turtle, who later passed it down to Avatar Aang, who used the ability to remove Fire Lord Ozai's firebending forever.
The capabilities of energybending are not well understood; what is known is that bending a person's life energy can impart them with knowledge instantly or remove one's bending completely. This bending art is dissimilar to the others because it is so perilous to the user, which is why so few have ever used it.