just like riding a bike

If something is just like riding a bike this means that even if you have not done it in a while, you wont ever forget how to do it. Riding a bike is sometimes hard in the beginning but once you learn how to do it, it becomes very easy and riding a bike is often related to something which once you learn it once, you wont ever forget how to do it. So when used in reference to something else, it means that if you learn it once, that is the entire lesson you need because you wont ever forget how to do it.