More specifically, Ansem was the ruler of the world Radiant Garden (also known as Hollow Bastion) and one of his apprentices, Xehanort, stole his identity and became a Heartless (the form a person takes after being consumed by darkness or otherwise losing their heart). However, Xehanort was already not his original self when this happened, having stolen the body of a young man named Terra beforehand.
You're welcome!
Welcome to Kingdom Hearts, bitch!

okay but once more, this time in english please.