You can use your coins to acquire Power-ups for your question streaks. As you respond to a question, you will find the “Bomb” and “Right answer” Power-ups at the bottom of your screen, along with their respective cost in coins.
In addition, you can use coins to avoid losing your turn, once you have exhausted your available attempts, and be able to complete your streak.
We recommend that you visit the article: Power-ups to get more information on these items.