What does it mean to have a bee in your bonnet?

If you were wearing a bonnet (a type of hat) and a bee flew in it, you'd be focused on that bee!

If you "have a bee in your bonnet" it means you're obsessed with something, usually an idea.
It means something that is really bugging you, or that you are really passionate about. You can't get it out of your mind. It is a "bee in your bonnet".
Having a 'Bee in your bonnet' means to have something to prove or a mission. Typically it means they have something they want and they are going to get it and they will steamroll others in the process.