First, press the green flag. To move the red square, use the arrow keys. To move the blue square, use the mouse. The goal is to get as high of a score as you can. The green circles can kill the red square, but not the blue square. The blue square is used for collecting the yellow dots, which can increase your score. If you survive long with the red square, it will also increase your score.
Eat pie I did this for school, so I know it's bad.