It means you are unable to stop thinking about something (or many things). You continuously think about whatever it is, and can cause you to feel like you are not able to relax. Said another way - You could simply refer to that thing (whatever topic it is) and say that you can't stop thinking about the test you have to take. Or maybe an issue in your relationship that keeps replaying through your mind uncontrollably, and you're not able to pause, and collect your thoughts to think about it in a calm manner. That's my two cents.