A century prior to the series’ opening, when Aang was 12 years old, he was frozen into a state of suspended animation for 100 years. After being awoken by two children from the Southern Water Tribe, he finds the world at war with the Fire Nation. As the Avatar, he sets out to master the elements and ultimately defeat the Fire Lord and end the war. Each generation yields one person who is capable of Bending all four elements. This being is referred to as the Avatar, the spirit of the planet manifested in human form. When an Avatar dies, they are reincarnated into the next nation in the Avatar Cycle. The Avatar Cycle parallels the seasons: winter for The Water Tribes, spring for the Earth Kingdom, summer for the Fire Nation, and autumn for the Air Nomads. Legend holds the Avatar must master each bending art in order, starting with their native element. This can sometimes be compromised when the situation requires it, as Aang demonstrates in the show. For the Avatar, learning to bend the element opposite their native element can be extremely difficult. This is because opposing Bending arts are based on opposing fighting styles and disciplines. Firebending and Waterbending are opposites, as are Earthbending and Airbending.
Avatar: The Last Air Bender Chapter 02