This feature will reveal one random letter in the crossword to help you form a word. This option will be unlocked at Level 5. To use a Hint, simply tap the light bulb icon that can be found on the right side of the game screen. 

It will be free on the first use. If you wish to use the feature again, you will need to purchase it using Coins. You can also sometimes get a free Hint as a reward from completing a Puzzle. 

A Hint can also be earned by completing a puzzle and opening a Bonus Gift.