To start the pong just click or touch on the left paddle which is human. Right paddle is Robot.
Start moving your mouse or on touch devices just move your finger to move the paddle. You will see the updated score on top of the game.
Whenever human or robot looses the game you will see the update on bottom of the game and you can start by clicking on paddle again. If you want to reset the level in between, just click anywhere in between the box. The same level will restart without updating the score.