The collections are groups of characters that you'll obtain as you level up in the game. To get characters, you'll need to obtain crowns in the different game modes.
By completing each collection, you'll be rewarded gold bars and two powerups. Keep in mind that powerups will be unlocked progressively; as soon as you complete the first collection, you'll get two of the available powerups; as soon as you complete the second one, the two remaining powerups will become available. As you complete the following collections, you'll get different powerups for free in the different game modes and an increasing number of gold bars.