You've got five random checkouts to unlock and finish within three darts each.
If you choose to play professional mode and fail to finish a checkout, you're back at the first again.
With the other options you have 3 or 5 lives and lose a life for each missed checkout.
When lifes are down to zero you're back at the first checkout again.
The game continues until you finished all 5 checkouts or press the X top right to end the game.
If you just leave the game, it will be treated as unfinished and cancelled.