Crystals are your in-game currency which you can use to buy anything that you need. But you can still use your matching skills to get bonuses and boosters. 

You can use crystals to do the following:

1. Buy bonuses, boosters and coins
2. Buy extra moves on match-3 levels
3. Refill lives

You can get crystals the following way:

1. Win match-3 levels
2. Complete chapters
3. Buy crystals in the Bank