Why is it hard to function after brain surgery?

My little cousin had a 14 hour procedure to remove a tumor next to her pituitary gland. its been two days and she is just starting to eat on her own, she sort of talks and the doctors are encouraging us to stimulate her brain. Why is it that this happens? I'm 100 miles away so cant ask but is it normal for someone to have to learn how to walk and talk again? she kind of talks but is mostly out of it. Does the brain like...reset itself or something? I guess I just dont understand how one can go from normal function to having to learn how to do basic things again, what happens during surgery to cause this? and is it likely she will get back to normal?