Collections are groups of characters that you will obtain as you increase your level in the game. To get characters, you must obtain crowns in the different game modes.
By completing each collection you will get as a reward two power-ups and gold bars. Keep in mind that the power-ups will be progressively unlocked; as soon as you complete the first collection, two of the available power-ups will be enabled; and as soon as you complete the second, the remaining two will be enabled.
As you continue to complete the following collections, you will get different power-ups to use for free in the different game modes and an increasing number of gold bars.
You will be able to see your collections by selecting the character that is, at that moment, on the main screen of the game.