It's pretty self-explanatory really. It disables auto aiming when firing a bow (or crossbow) so the arrow (or bolt) goes where you aim, and not where the game wants it to go, which drastically decreases the chances of the arrow missing its target for multiple reasons derived from auto-aiming (multiple enemies, target partially behind cover/obstructed, etc).
I've been using it for, like, forever. The only 'problem' with it is that sometimes when the game triggers a killcam, and the target moves, the arrow will actually miss the target, because it moved. You can look at it as a bug, or as a feature (more realistic) :P
EDIT: Apparently some ini tweaks from STEP already take care of it, sorry.