What does it mean to truly have free will?

Your brain is billions of electrically-reactive cells and interconnections, and your consciousness is electrical impulses shimmering around those cells in a more or less stable way.

A belief in free will implies the belief that you essentially have control over the physical mechanism that makes up you. A brain may not be a fully deterministic machine in the sense that a certain input condition may not always result in the same output, but it seems like a stretch to say that "you" both directly result from and directly control that mechanism.