What does the idiom "a penny is a lot of money if you haven't got a penny" mean

Everything is relative. If you're broke and someone hands you 20$, it will seem like a lot of money to you. However, if you hand the same amount of money (20$) to Bill Gates, it will seem like a small amount of money to him. A penny does not have a lot of intrinsic value, but to someone who has absolutely no money, it will seem like a lot (relatively speaking).