When you have started the game you press space to start a pattern of colours and sounds. When you see/hear something that doesn't fit the looping sequence you press space again and your score will increase and there will be a new pattern for you to do the same on. If you press space when a correct colour/sound if shown you get a game over and the game stops, showing you your score. Press space again when the game is over to restart. Get as high of a score as you are able to.
Hope this helps :)