Gems are the currency of the game.
You get some at the beginning for free, and you can gain 50 more gems for signing up and creating an account but after that they must be purchased.
You can purchase more gems and special packs at the store.
Gems are used to buy extras--extra hearts, extra moves, extra time, and extra tries at guessing letters. They can also be used to buy more boosters, both in-game boosters and starting boosters. You can spend gems in the moment you need them by either following the prompts when you run out of lives, moves, time, etc.
You can also spend gems during games by tapping the symbol for a booster you want. If you already have that booster, it will activate the booster. But if you don’t have that booster, you will be asked if you want to purchase some.