Essentially it means favoring that side of the pocket such that missing the shot leaves a more difficult situation for your opponent. Say a spot shot where the CB is on the head spot and the OB is on the foot spot and you are shooting the OB to one of the foot corners. Missing on the pro side in this situation would be the over-cut. In this way, if you over-cut the ball and miss, the OB goes to the side rail and with the right speed ends up in the middle of the foot rail, with the CB coming back to near the head rail somewhere. This leaves a difficult situation for your opponent.
If you had undercut the shot, the OB hits the foot rail first and then comes up towards the middle of the table, most likely leaving an easier shot for your opponent.
There's a thread around here somewhere. I'll see if I can find it.