Every week, you can complete a set of missions and collect keys to unlock special reward cases. To view these missions, tap the Missions tab on the Main Screen. Please note: the timer just below the word "Missions" shows how much time is left until your tasks and progress towards case rewards are reset.
Completing a mission gives you keys, and you can see how many keys you've collected so far by looking at the keys bar at the top of the Missions window. On that bar, you can also see the case rewards, as well as their key requirements. To view contents of each case, simply tap on them.
After you complete a regular mission, that task slot starts a cooldown timer, which you can remove by watching ads. After this cooldown runs out, a new mission takes the slot. Premium missions do not have a timer, so you can complete them at your own pace.
Once you collect all case rewards, you can refresh your progress with gold. This will drop your keys counter back to 0, and you'll be able to earn cases again. However, please remember, that your progress gets refreshed automatically when the weekly timer at the top runs out, so plan your missions accordingly!