A bail is a fitted metal loop or ring at the butt of a knife, usually a small D-ring or shackle, for clipping the knife to a lanyard, belt loop or carabiner. Sailors and rope workers use them constantly.
A lanyard hole is a hole through the handle that you thread cord through. A bail is a separate metal fitting that swivels or hangs, so you can clip straight to it without tying anything.
The practical difference is speed. A bail accepts a carabiner or snap hook instantly; a lanyard hole needs cord installed first.
Marine and rigging knives almost always have one, because a knife dropped off a boat is gone and a knife tethered to your belt is not. Rescue knives and some heavy-duty folders carry them too.
The downside is bulk and noise. A metal bail adds weight at the butt and can rattle or catch on things, which is why most EDC folders offer a lanyard hole instead.