This sentence is quite tricky. It means that you can't have more than what you can handle or trying to have 2 incompatible things. It's like saying that you can only have something done in one way or the other. I mean, once you eat your cake, you definitely won't have it anymore since you've already eaten it. Let's say that you want to buy a book but don't want to spend some money. In this case, you can't have your cake and eat it too, since you have to spend some money if you wanna buy something. hope this helps.