If we have free will, we can consciously make decisions that are not determined by the physics and biology of our brains. It's a philosophical and religious concept that has found no support in science, despite the strong illusion that we are free. Even if we are not, belief in free will can be healthy, as long as we remain aware that many factors influence our behavior subconsciously. 
Free will is the idea that we decide for ourselves what we do.
- If we are free then can everything that happens be pre-determined?
- If we are not free then can we be held responsible for what we do?