A hidden code of four coloured pegs is waiting behind the rack. Guess a row, lock it in, and the board answers with two numbers: how many pegs are the right colour in the right place, and how many are the right colour in the wrong place. It never tells you which is which — that is the whole game. Ten rows to get there.
Colours can repeat unless you turn that off, which makes 1296 possible codes. Click the colour buttons or use the number keys, backspace to undo, enter to lock in. When the game ends you get a breakdown of how many of those 1296 each of your guesses actually ruled out.