Collections are groups of characters that you receive as you level up in the game. To obtain characters, you will have to acquire crowns throughout the various game modes.
For each collection that you complete, you will be rewarded with two power-ups and gold bars. Bear in mind that the power-ups will be unlocked progressively; as soon as you complete the first collection, two of the available power-ups will be enabled, while the remaining two will be available once you complete the second collection.
As you complete the following collections, you will receive different power-ups to use in the different game modes and an increasing amount of gold bars.
You will be able to view your collections by pressing the character currently on the main screen of the game.