Imagine you are having breakfast and sitting on your breakfast table. Everything on your table and reachable w/o getting up is what your CPU holds in its register. When you need something from the fridge in your home, this is your RAM. If you need something that is not in your fridge, you have to get dressed, get out of our home, walk to the groceries store which is half an hour away, find what you are looking for, pay for it, walk home for around half an our, switch back to your relaxed clothing and finally you can continue your breakfast. The groceries store is your hard disk/ssd whatever. With compression, imagine you have a big second fridge in the basement (or the house next to yours, you get the idea). Not as good as having stuff on your table or in your fridge, but usually at least an order of magnitude better than having to visit the groceries store.