Karma means the effects of all deeds actively create past, present, and future experiences. Basically they belief that you effect your own life. There are variations of course, but the idea is you'd get what you deserve. Nasty to people in your life, then you shouldn't end up living in paradise by the end of it, is how I've come to understand it.
No, you're right, it's not. But it does happen. At the worst of times, it feels like.