A lanyard hole is an opening through the handle that you thread cord through. A bail is a fitted metal loop or ring at the butt that you clip to directly. Both exist to secure or retrieve the knife; the bail is faster to attach to and the lanyard hole is lighter and quieter.
Simply a hole, usually at the butt. Thread paracord through it, tie a knot or add a bead, and you have a pull for retrieving the knife from a deep pocket, a tether against loss, or a little extra bulk to fill the hand on a small handle.
It adds no weight and cannot rattle. See lanyard hole.
A metal D-ring or shackle fitted to the butt. Clip a carabiner or snap hook to it and the knife is secured instantly with nothing tied. That immediacy is why sailors and riggers use them.
It costs weight at the butt and can rattle or snag. See bail.
For everyday carry, a lanyard hole. For work over water, height or anywhere a drop is unrecoverable, a bail.
One safety note either way: do not use a lanyard as a wrist loop during hard cutting. If the knife slips, being tethered to it is worse than dropping it.