Simple usage: "I want to help you prepare for the test but I am not sure if I am able to". For example, you have a mathematics exam. If the subject is simple arithmetic, I can help you to learn how to do the questions, but if it's about something more difficult then I can't help because it's too difficult.
There are more complicated ways of using "might be able to..." but they are more unusual and more difficult to explain. Usually, the person really means "I might be willing to help you if you will do something for me" but they don't want to say that.