Howard Hill popularized the term. I may not explain it real well, but basically, your primary vision and concentration are on "the spot"... the usual advice of "pick a spot"... while with your peripheral vision, you're kind of gapping, basically being aware of where you arrow point is in relation to that spot.
Try this: stand up and look at a closed door across the room. Concentrate on the door knob, burn a hole in it with your eyes. Now raise your hand and, without changing your concentration, point your finger at the doorknob. Your peripheral vision will let you be aware of your finger. Since your arm is lower than your eye, there will be some angle to your arm, similar to what your brain and eye perceive when shooting the bow.
If you train yourself to do this, you can practice at different ranges and become aware of where your arrow point is in relation to the "spot" at different distances. It will be below your spot at shorter ranges, on the spot at your "point on" range (usually around 30 yards, but depends on your bow, draw and form), and above the spot past your point on. Memorize your distances and practice accurate distance estimating and you can get very consistent about it.
Bob Wesley adds the trick of memorizing the height of various animals knees and hearts so that you can immediately know where you want to put your arrow head for the distance you estimate you are from the animal.
Sounds a bit complicated, but it works well. Other systems work well, too. The main thing is to pick the one that works for you and make it yours.