A lot of smart people have trouble getting into college because they don't do anything but study. They have very high grades, and they have top SAT scores and other things that make them look very desirable. However, they haven't done anything else, which makes the college question whether they have any other interests or whether they are capable of handling more than one thing at a time.
It can hurt your chances of getting into college if you only study and you don't do anything else. That doesn't mean that you have to be a cheerleader, sports player, captain of the debate team, work on the school newspaper and tutor underprivileged kids in your spare time. Some people try to do too much, and then their grades slip, which can also keep them from getting into the college of their choice.
If you have no extracurricular activities, though, you'll want to pick up at least one or two throughout most of your high school career in order to have the best chance of getting accepted at the college of your choice.
