There are a few reasons why the TCS light might be on. One reason is that the system has detected that the wheels are slipping. This can happen when you are accelerating too quickly, driving on a wet or slippery road, or turning too sharply. Another reason why the TCS light might be on is that there is a problem with the system itself. If you are not sure why the TCS light is on, it is important to have your car checked by a qualified mechanic.