A Half-Long or Mid-Long serve is one whose second bounce is right at the end of the table. This is an effective strategy because the receiver has to make a quick decision whether to play the ball early, over the table, or wait for it to pass the end line. The player will sometimes make the wrong choice, and may try to loop the ball, but the ball may double bounce on the table.
These serves should be kept very low, as a high ball, even if it double bounces, can still be attacked by a good player.
At the professional ranks the half-long serve is very common.
Counterintuitively, a very short serve close to the net is often not a good choice. Players who are good at the Flip/Flick will be able to attack a short serve, even if it is very low.
These are sub-pages of the Half-Long or Mid-Long Serves page (a child in the hiararchy).