Compatibilism is a theory that is meant to reconcile the idea of free will with the idea of determinism (the idea that our fate is already determined).
Many people believe that these two things cannot be compatible. If our fate is already determined, we have no free will because what is going to happen to us is already determined.
There are many different strands of compatibilism that solve the dilemma in different ways. For example, classical compatibilism argues that people are able to choose what they do. They are not (so far as they know) forced in one way or another. Because they are not forced, they have free will. Someone may know what they are going to do, but they themselves do not know.