The numbers say how many squares in a row are filled in that line, in order, with at least one empty square between runs — 2 3 means a run of two, a gap, then a run of three. Fill the squares that must be filled and cross the ones that must be empty; a line's numbers go grey when it is done. When every filled square is right, a picture appears. Check marks any filled square that is wrong, and counts as help.
Keys: arrows move, Space or Enter fills, X crosses, F and X also switch the tool, Z undo, Y redo, C check. On a mouse, the right button crosses.