The Avatars are bending arts masters who are chosen by the Gods and are the only bender who can master all four elements. A bender was native from either The Air Temple, Fire Nation, Water Tribe or Earth Kingdom. Which ever nation they’re indigenous to already know that element and must travel to the other 3 nations to learn the other elements. The current Avatar Aang who was The Last Airbender was destined to restore balance to the world by defeating The Fire Lord.
They many afters like Avatar roku and later sometime after the events of Avatar the last Airbender Korra the titular character from the Legend of Korra series is now the current avatar.