Who hasn't played pong? You are playing against the Enemy in this game of 2D computer tennis. The goal of the game is to get the ball through the opposite side without the Enemy sending it back. One point is awarded for scoring. If the Enemy gets the ball past your paddle, then it receives a point.
Whoever scores three points first is the winner of that level. If you win, you advance to the next level where it gets a little harder. If you lose, you're back to the beginning.
There are only four levels, but if you can keep winning, you can keep playing.