You are actively trying to change how you think. It's not necessarily thinking about nothing. In many cases it's about coming at the same thoughts from a different perspective, often an external and non-judgemental one.

It really depends on which tradition or practice of meditation you're talking about. In the exercise I go through, it's different because it's specifically focused on acknowledging whatever thoughts come through without judging or reacting to them like I would impulsively. Being able to accept especially the negative thoughts at face value helps me a lot with managing the coping process.