I think he means cutting the nail too short. So the guard stops the nail from going in too far and cutting the nails so short it hurts the nail bed underneath.

Personally I like my nails to be on the short side, since I do things like machining and woodworking as a hobby. Nothing ruins a fun day of working in the shop as folding or splitting a nail in the first 5 minutes. But I can understand for people who like to have their nails a bit on the longer side if this isn’t a concern.