Original: The series takes place 100 years after the fire nation declares war against all other nations and has killed off all air benders in search of finding the avatar who has been reincarnated as an air bender, Aang.
Simplified: The series takes place 100 years after the Fire nation started a war against all other nations and has killed off all air benders in search of finding the avatar who has been reincarnated as an air bender, Aang.