Can someone explain "free will" to me?

Admittedly, I am rushing this one, and I’m having a difficult time wording it. Most of my difficulty is due to my inability to truly understand this.

We can identify that we are the product of our brains when we consider someone who experienced a brain injury. Let’s say that Bob injures his brain in an accident? What if Bob no longer enjoys the company of his wife or kids? What if he has new desires – and acts on those desires? What if he engages in illegal activity?

Will we describe his actions as “free”? More than likely, we’ll point to his brain injury. But if Bob is simply the result of his brain, are we not also products of our brains? If we were all born with different genetics and exposed to different experiences throughout our lives, and if this is what influences our decisions, where does the concept of a “free” action enter the picture?

And before you say, “yes, but Bob is clearly injured – you and I are not”...
But brain injury is relevant here only to point out how the brain affects our processing and decisions.

If I had the brain of someone who murdered his kids, I would murder my kids, right? That is, what other variables are at play here?

So, what could it possibly mean to have “free will”?

I apologize for this messy question.