Finish the checkout within 9 darts.
For each finished checkout the next checkout will be increased. If you finish the checkout within 3 darts, that checkout is also locked and is your new base checkout. If you don't finish a checkout within 9 darts your are back to your base checkout. Play as long as you want or if you reach 170!
If you want to quit the game earlier - press the X in top right to save your score!
Game explained on Youtube: