Coins are Quizcover's internal currency. If something you might want to buy costs coins, the price is typically shown on a (typically red) badge on a button.
You get a limited quantity of Quizcover coins initially, and you can always purchase more in the form of coins packages in the Store:
You need an online connection for purchasing and spending coins. However, if you buy booster packages with coins, you can use your already-purchased boosters in offline mode.