The judicial branch checks the other branches of government by the fact that it is able to say that things they have done are unconstitutional. When it does this, it prevents them from doing things that they are not allowed to do by the Constitution.
The Constitution sets various limits on what the government can do. For example, the government may not make laws that infringe on freedom of religion. However, it is possible for the legislative branch to make a law, or for the executive branch to write a rule, that does infringe on people’s freedom of religion. In this case, the judicial branch is able to check the other branches by ruling that their action is unconstitutional and therefore has no force.
The judicial branch does not do this very often, but the fact that it can do so acts as a check on the elected branches of government, making them have to be careful of what they do.