15:10
This can be written in several ways, but most commonly it is written as 15|10 here on Chess.com. It means that you begin the game with 15 minutes on your clock (as does your opponent). After each move that you make 10 seconds will be added to your clock. Your opponent also gets the same added after each of his/her moves. This is usually called the increment. So if on the first move you take 15 seconds to make your move, your clock will then show 14:55 (15:00 - 0:15 + 0:10).
If you need help, please contact our Help and Support team.