The game is played by guessing what four digits are behind the question marks. Enter a guess and click the check button. The game will evaluate your guess and respond by placing an open circle for each guess that is a correct digit that is out of place and a solid circle for each correct digit that is also in the correct place.
For example if the digits 1, 2, 3, 4 are behind the question marks and I guess 6, 4, 3, 5 the game will give me one open circle and one solid circle. The 4 is a correct digit, but not in the correct place. The 3 is a correct digit that is also in the correct place. From this information I can place another guess by changing one or more of my guesses and clicking check again. By reasoning and elimination try to guess the correct digits in as few guesses as possible.
See Mastermind board game for more information.