Co witness means the red dot would be at the same spot as the iron sights. Like if you were to look through the iron sights, the red dot would be in the same spot.
1/3 co witness moves the optic up higher so the sight you see through the irons, would be 1/3 of the way up on the red dog's display
If that makes sense?
If you are far sighted and wear glasses for up close, I find it easier to take your glasses off, since you are looking at the target, through the sights. With progressive lenses, you'd want to be looking through the distance part of them when using the sights.