Everyone at some point is bothered by an unwanted or intrusive unpleasant thought, but for some people these thoughts are excessive and can cause distress and anxiety. These thoughts then become obsessions. Many people who have obsessions will try to control or suppress these thoughts, which can be almost impossible.
What the obsessive thought is about will vary from person to person: One person might be obsessed with the thought that they could get very sick another by the thought that something bad may happen to a loved a one.