First click on the applet. Then:
Pong in 4D!
The hypercube playing field exists in four dimensions: x,y,z,w. Since it's a hypercube, the playing field is bounded by eight hyperfaces, each of which is a cube. Every time the puck hits and bounces off a hyperface, the edges of that hyperface light up.
Six of the hyperfaces are in the positive and negative x, y, and z dimensions. When the puck hits one of these hyperfaces, the edges of that hyperface glow white. Essentially, these six hyperfaces are the walls that the puck can bounce off on its way from one end of the playing field to the other.
The two hyperfaces in the positive and negative w dimension are special. They are the near (red) and far (blue) hyperfaces. You are hitting the puck from the red toward the blue, and your opponent is hitting the puck from the blue toward the red. Your job is to move your red paddle in x,y,z to block your opponent'ss shot.