Free will is when you are able to do whatever you want freely. Free will can be thought of as a philosophical idea - whether we decide what we do, or if nature, destiny or supernatural forces control us. It can also be thought of in a legal way - whether or not you can legally do whatever you want. Sometimes people do not have the legal right to do what they want.
Free will presents a problem when you think about it. One way to see the world is that physics tells us what will happen if we do something. If we throw a ball up, into the air, the ball will fall back down. If the world was only physics, then all of our choices are controlled by past events. This means that our choice to throw the ball or not is determined by some past event. (See determinism) The other way to see the world is that we actually decide what we do. The past does not make us choose something.
Most philosophers[source?] find ways to think about free will so the problem goes away. Some[source?]think we do not have free will--the past determines our choices. Others[source?] think we have free will--we choose to do what we do ourselves. Some[source?] find ways to mix these two views.
The law assumes we have free will. The job of courts is to find out when people do things and what they were thinking when they decided to do it. For example, think of someone who kills someone else. A court tries to figure out (1) if she actually killed the other person, and (2) if she decided to do it. The courts do not ask the philosophical question above.