What does it mean that the stack is a LIFO structure?

LIFO stands for Last In First Out. It basically means that the most recent piece of data added to a list, will be the first piece that is taken off. It is often compared to a stack of plates. When you have a plate to add to the the stack, you put it on the top, not in the middle or at the bottom of the stack. If you need a plate you usually take it from the top, not the middle or bottom. So the most recent plate on the top is the first that will be taken off when one is needed.