You start filling the holes at the bottom row with pegs, and each time you have completed a guess (i.e. filled all four holes) the computer will respond by giving you one peg for each correct color - black if your guess was also in the right place, otherwise white. (But you will not be informed about which colors were correct. That part you will have to figure out for yourself.)
If you haven't managed to guess the sequence by your 8th guess, the computer will reveal it to you and the game ends.
First select which colors you want to play with. (The default set contains blue, green, red and yellow. That's moderately difficult. For obvious reasons you are not allowed to select fewer than two. :-) Clicking on a color in the field on the right will toggle the circle around it on/off, indicating whether it will be part of the set the computer draws from. Then press 'Start' to begin.
In case you wish to start over, just press 'End' to discontinue the game.