As you complete quests, you'll receive various items that can be assembled into collections. Each collection consists of the collection items themselves, chargers and the final reward.

Chargers are items required to turn a collection in. Without them, you won't receive the final reward. You can find chargers by trading with characters, or you can purchase them for crystals.

The final reward is an item you get after turning in a collection. These items are usually required to complete story quests.

As soon as you turn in a collection, you'll get a reward for doing so (besides the final reward). The reward for turning a collection in is listed below the list of collection items.

The list of all the collections can be opened with the Collections button at the bottom of the game screen. You can use the filter at the bottom to see new collections, the ones that are ready to turn in, or the ones that have 4 out of 5 items.