Formal social control is done through laws, rules, and other such official things. If you disobey formal social rules and laws (and you are caught), you are punished in formal and legal ways. This is in contrast to informal social control which comes about when other people act in disapproving ways when you break unwritten social norms.
Formal social control can be done by laws. If you are caught speeding, you will be sanctioned through a fine. It can also be done through the rules of organizations to which people belong. For example, if you are caught breaking the formal rules against plagiarism in the academic world, you might receive a failing grade on an assignment or for an entire course. You might possibly be expelled from school.
There are two main reasons why this might not work. First, there is not always the certainty that you will get caught. There are millions of people driving every day and only a relatively few police officers. If you speed, there is a very good chance that you will not get caught. Therefore, the formal law against speeding is not very effective. Second, and perhaps more importantly, formal laws are not necessarily things that we agree with. Therefore, we do not feel any moral qualms about breaking them. If you do not think that it is wrong for you to download copyrighted music off the internet, you are likely to do it even though there may be sanctions against you if you are caught.
Thus, formal social control is not completely effective both because people are not always afraid they will be caught and because they do not always feel any internal pressure to comply with the formal laws and rules.