'Reveals' give you the opportunity to reveal the correct answer for questions you have answered incorrectly.
You can use three reveals per hour. The number above your avatar shows how many you have left for that hour. They will be refreshed the following hour.
When you get a question incorrect it will ask if you want to reveal the correct answer
If you click the correct answer button you will get a prompt that will remind you that you are using one of your reveals. If you choose unlock, voila! you will see the correct answer!