The term “law enforcement” is generally used to refer to the police and to other such official entities. These are entities whose main role is to ensure that the laws of the society are adhered to. So, we can say that what law enforcement does is to make sure that people are obeying the laws of society.
More specifically, we can say that law enforcement does at least four things. First, they try to discover when crimes are happening. That is, the police sometimes patrol around looking for criminal activity. Second, they try to identify the people who have committed crimes. This is the sort of thing that is typically seen on TV shows. The police know that a crime has happened, and they try to determine who committed the crime. Fourth, they try to apprehend people who have committed crimes. Sometimes, they do this when they see the crimes happen themselves. At other times, however, they try to apprehend people when they have determined through investigation that those people are guilty. Finally, the police sometimes try to deter crime. They do so by patrolling and through other such activities.
Thus, law enforcement typically is concerned with making sure that the laws are obeyed.