To open a case in CS2 you need to own a key. You can buy a key in CS2 or on the steam community market. Then you press "UNLOCK CONTAINER" to open the case. Usually buying the keys on the steam community market is way more expensive than buying them in-game. Keys are not as cheap as cases, they cost around 2,25€ which equals 2.50$. When you have a key, you can open one case with it. After opening the case, your key and your case are gone and you get a skin in return. However, you have to be careful, keys only open one type of case. For example, an Operation Breakout Key can only open an Operation Breakout Case. To open a Chroma Case you need a Chroma Key and so on.