This is an English Idiom. It means to have an advantage over someone else in different areas of life. Example, She probably has a leg up on the other applicants for the job because she has more experience.

Imagine a race up stairs or a mountain... If you are barely in the lead, you may have only one leg ahead of the rest. That is how you should imagine it. Here is how you use it... You have some edge that will give you a better chance. Because of my tutor, I have a leg up on this test. (and it may mean you have and advantage over yourself if you had no teacher, or the other students if you are competitive with your grades - but you say "this test." This new radar gives me a leg up on the cops. I have a leg up on the others who want to stay up all night, I took a nap.

Generally it is used to state an advantage over something. For example: "By doing my homework early, I got a leg up on today's lesson." I hope this helps!