Drag a piece or click a move in the panel on the right. Pawns reaching the far rank become queens unless you use the panel to ask for something else, and castling is done by moving the king two squares towards the rook. The computer plays the other side and thinks for about as long as you would want it to.
It is a plain board with no clock and no rating, and that is the appeal: nothing is being tracked, nothing is at stake, and you can take the whole thing back to move nine and try the other bishop. Good for a lunch break, and good for the particular pleasure of beating a machine that is not trying very hard.