In Hindu mythology, gods manifest themselves into Avatars to restore balance on earth, usually after a period of great evil. The Chinese characters that appear at the top of the show's title card mean "the divine medium who has descended upon the mortal world."

In order for Aang to defeat the Fire Lord and restore balance on earth, he needs to master all four elements, air, fire, earth, and water. As the title says, Aang is the last air bender, meaning he can control air, and use it anyway he likes, but he still has to improve his air bending.
In the Avatar, the world is divided into four nations, each according to the four elements, Earth nation, Fire Nation, Air Nation, and Water Tribe. And luckily Aang
found friends who could teach him all four elements, from each country. Katara, from the Water Tribe and her brother Sokka, found Aang, so Katara began to teach Aang water bending, because, Katara was the only one alive in the water tribe who knows water bending, the others and her brother Sokka, doesn't water bend. But as the story progresses, Katara was able to meet an old lady banished from the Water Tribe who knows water bending and taught her to master the tech
niques, and introduced to her Blood Bending, which has the ability to turn humans or anything that has blood into her puppets! (alright! that's awesome!)
When they got to the Earth country, called Ba Sing Se, Aang found an Earth bender, who also has the ability to bend metal, she's the only one who could do that from the Earth country, and guess she's blind! She calls herself the blind bandit when Aang was looking for her, she's Toph Bei Fong. She's pretty annoying in the beginning but as the story changes, so she does, she's really kickin' ass!
And the last but definitely not the least who joined his team, and became his fire bender teacher, is Prince Zuko, Fire Lord Ozai's one and only son, whom he banished from the Fire c
ountry because he's a disgrace to the empire. Prince Zuko wasted his effort on defeating the Avatar to regain his honor, but later realized that his honor could not be given by anyone, not even killing Aang could regain his honor, with his uncle's help, he changed into being a good friend of Aang. Though it was kinda difficult for them to accept Prince Zuko in the team, knowing that he's the son of the Fire Lord, he e
arned their trusts and taught Aang fire bending!So when the time arrived for the Avatar to face the Fire Lord, it was one awesome duel, hope you could see the episodes too. If you want to you can go here. You can watch the episodes there via streaming. Aang, doesn't want to kill people, so he didn't kill Fire Lord Ozai, but removed his fire bending and locked him in the dungeon of his own country. Then, guess who took the throne, Prince Zuko!