Speaking philosophically, free will means that a person's actions are determined by choice and not by a predetermined set of circumstances, a fate, or a higher power.
If you have free will, then you are in command of your behavior. If you don't have free will, then your actions are decided already, either by the circumstances of your biology, your society, or something else.
Free will is fundamental to our assumption of justice and law. If we believed, truly, that people were not in command of their own decisions, we would have to change the way we view right and wrong.
How could we blame Mr. X for stealing a car if he was predetermined to do it and had no power over that decision?
Science suggests that there is probably a bit of both (determination/behaviorism and free will) at work in all of us. We may be hard-wired to choose certain actions. We may be socially predisposed to act certain ways also. Yet, in the end, there is something in us (our conscience) that agrees or disagrees with an act before it is taken.
Certain disorders complicate this notion. But, mostly, that is how our culture views the motivational nature of man.
Free will is the power to choose. Having choice means that no outside force can (or will) make choices for you. Possessing free will is as elemental as breathing. Without it, a flourishing individual cannot survive, but is reduced instead to an automaton whose decisions are enforced by some external authority.
As the answers above suggest, free will is the ability to make independent choices. Some philosophers argue that there is no such thing as free will -- that every choice we make is determined by numerous forces that dictate those choices. For these "determinists," free will is merely an illusion.
If we didn't have free will, we would not be human. Free will means being able to have moral choices as to what you are going to do. It means that we can understand the moral consequences of what we do and we can choose what to do based on those understandings. This gives us the unique ability to be morally good or morally bad.
Free will is being allowed to make your own life's descisions yourself. We relate everything to our daily lives. Unfortunateley not all people of a society are given their right of free will.