Loot Chests, available in varying sizes and with different contents, contain rewards, including Cards, Gems, & Coins, and are earned through victories in battle or can be purchased directly from the Shop.
In general, you can only unlock Cards available at your current and preceding Ranks. Loot Chests obtained through victory take a certain amount of time before they can be opened and the rewards subsequently claimed. Only one of these boxes can be opened at a given time, but Gems can be used to open it immediately. Chests purchased through the Shop are opened immediately.