"under the net" meaning that the ball has passed onto the other side of the court under the top of the net cord. usually this happened when you played the ball around the outside of the net post to drop the ball over, but the ball had already dropped below the top of the net.
according to the rules, the ball still is good even if it goes back to the other side around the outside of the post as long as it passes over the top of the net cord and then hit the ground again inside the line on the other side of the court.
this means that the top of the net actually extends to both sides indefinitely, which is unlike in volleyball. That is why the volleyball net has two poles extending upward at the end of the net whereas there is none in tennis.
if you watch the professional players, they very often get pushed to the extreme far side of the pitch and they would return the ball to the other side so that it would land right at the corner of the other court, winning the point.