A social construct is something that has no objective existence outside of what human beings have made of it. It is different from an objective fact that does not need to be defined by human beings.
In the context of crime, let us look at what this means. Death is an objective fact. We do not make up the idea that a dead person is different from a living person. This is not a social construct. But the idea of murder is a social construct. We have made up categories of killing that say when it is acceptable to kill and when it is not. This is a social construct because it does not exist independent of human beings making it up.
Our approach to crime is very much a social construct. We define certain actions as legal or illegal. We then assign different punishments to them on the basis of how bad our society deems them to be (a social construct). For example, we typically give a harsher punishment to someone who steals a little bit of money by threat of violence than to someone who steals a lot of money through fraud. This is an example of how our approach to crime is affected by the social constructs we create around the issue of crime.