As the name suggests, the name of the game is to turn out all the lights. This is done by pressing the lights. When you press a light, that light and the four adjacent (vertically and horizontally, but not diagonally) lights will toggle their on/off state. (Have a little play with it -- you'll see what I mean.)

There are 50 levels, and each level has a minimum number of moves required to finish the level, as shown below: The buttons behave as follows: