It refers to playing the percentages and not selling out. As others have stated, a two way shot. If you have a long tough cut shot on the game ball that you know there’s a decent chance you might miss, You tell yourself to make sure you do not undercut shot. You play the shot at a speed which will allow the cue ball to come all the way back down near the end rail and the object ball if you overcut it, to end up somewhere on the opposite end rail, leaving a tough shot for your opponent.
Good bank players do the same thing on bank shots - choosing to err on undercutting the bank just a hair and controlling the speed, so that if they miss, most likely the object ball stays very close to the end rail and leaves the cue ball somewhere at the other end of the table.
Yes, there is certainly the possibility that you could overcut the shot just enough to rattle it in the pocket or hit the point and leave it close enough to the pocket for your opponent to make, but it is still a percentage play that, over the long run should increase your chances of winning the game.