A ball which clips the net and bounces inside the service box is known as a 'let'.
If this happens the player is allowed to serve again.
However, if the ball hits the net and lands outside the service box, it is a fault.
A 'let' can also be called during any point in the match if it seems fair for a point to be played again.
For example, if there is a dispute over a line call.
If the server throws the ball in the air but does not attempt a shot, it is a 'let'.
If the server throws the ball in the air, attempts a shot but misses, it is a fault.