There are two teams. These teams serve against each other. So you have to do the field vertically through the middle. On the picture you see what the left team has to do. The right team does the same but a little further to the right. The intention is to hit the person sitting down. This person has to put his butt on the ground and catch the ball without bouncing.
There is a person sitting on the other side of the table, just behind the net, on the right front side. The goal is to hit this person. You do this with your group until you succeed. If it succeeds, the server moves to the back right and the person in front of the net joins the row to serve as well. If the serve is successful and the person at the back can catch the ball without a bounce, the server may sit in the middle of the front row. If the server has a good serve on him, he will sit at the back mid court.
-->All positions are from the server's point of view on the other side of the net.
You can do that twice in a row, but you can also do that they have won if they have had all 4 positions. So it's a competition to see which of the two teams gets all 4 positions the quickest.