In a doubles game, on the second serve, the server serves the ball which hits the net cord and then drops into the receiver's alley. The receiver cannot reach it in time to return it.
The serve is a NET, which means replacing the serve. A LET means to replay the point, thus giving the server two serves again. This is the biggest mistake of words in tennis, not that it makes a whole lot of problems, but nonetheless wrong.
If the server serves the ball and it hits the net and still falls in the service area then it is not considered a fault and the server may repeat the serve.