The goal in this billiard drill is to use side spin on the cue ball to cause it to rebound to one rail and into perfect position for the next shot.
Billiards Position Play With Rails
In this pool drill, sometimes called the "one rail and out" drill, you try to pocket each object ball in the far corner pocket, using side english spin to make the cue ball rebound off one rail and back into perfect position for the next shot.
Set up four balls roughly a ball's width apart, just above the side pockets and perpendicular to the long rail, as shown in the diagram below.
To begin, start with the cue ball in hand from somewhere around position "A" as such that you can pocket the first ball in the far corner pocket. Use side spin on the cue ball to make it go to the cushion and rebound back into perfect position to shoot at the next ball.
With each shot, you need to come out further and further to get that perfect position for the next shot.
Here is a video from Billiard Boss showing the drill in action.
As mentioned in the video, the key is to not be satisfied with getting into any position with the cue ball, but rather, you should be satisfied only with getting into perfect position with the cue ball to do the next shot almost straight in. Listen to the video the whole way through. There is a lot of good commentary there.