How well does the depo shot work?

I got my shot today and I know it is 99% effective if used correctly but, what I don't understand is, what does it mean by correctly? Can he cum inside me or does that make my pregnancy chances higher? I know it doesn't protect against disease. I'm new to this and my doctor didn't really explain a whole lot except the information I mentioned above and that I need to get it Every three months. Thanks to those in advance.