Move your mouse cursor across the stage to control the paddle, and make the ball not touch the red line. This is the simplest game you will ever play and requires no prior experience, it's just ping pong.
By the way, there's absolutely no sound whatsoever in this game, I tried to make it work, but apparently, 10+ years of Scratch gets you nowhere. They probably fixed the sound altogether, but we'll see soon enough, but enough talking, just enjoy the game.