If you need help with a puzzle, coins can be used to purchase hints:

1. Reveal a letter ("?" icon): fills in the next letter in the currently selected word. 

2. Remove unused letters (star icon): Removes all letters from the letter tray that aren't used in the answer
 
Some of the game modes have different types of hints - please refer to the FAQ page for a specific game mode for full details.

These is no limit to the number of hints that can be used in a puzzle, provided you have enough coins to purchase them.

Coins can also be used to automatically complete quests or claim trophies.