Cases contain a variety of skins, animations, and melee weapon unlocks. Cases contain random skins varying from tier 1 to tier 7, depending on the type of case that you purchase. When a case gives you a duplicate skin, you will be compensated with blue tokens that you can use to buy skins directly from the loadout menu.
Cases are bought with credits from the in-game shop. Some milestones also grant cases as a reward.