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. It is standard on marine and rigging knives, where a dropped knife is a lost knife.
A lanyard hole is a plain hole you thread cord through. A bail is a separate metal fitting that hangs or swivels, so a carabiner or snap hook clips straight to it with nothing to tie.
The practical difference is speed. A bail is ready immediately; a lanyard hole needs cord installed first.
On a boat, on a ladder, or anywhere the knife falling means the knife is gone. Marine and rescue knives carry bails for that reason, and some heavy-duty folders do too.
Rigging and sailing knives often pair a bail with a sheepsfoot blade, which is a very deliberate combination.
Weight and noise. A metal bail adds mass at the butt and can rattle or catch on things, which is why most EDC folders offer a lanyard hole instead.
See bail versus lanyard hole for the fuller comparison.