The lock bar is the spring-tensioned piece of metal that moves across behind the blade tang to hold it open. On a liner lock it is a cut section of the inner liner; on a frame lock it is a cut section of the handle itself.
On a new knife the lock bar should engage early, sitting somewhere between a quarter and halfway across the tang. That leaves room for the natural wear that moves it further over across the life of the knife.
A brand-new knife that locks up at the far edge of the tang has very little wear left in it. That is worth noticing before you buy.
Lock stick, where the bar is hard to push back, usually settles with use or a pencil-graphite rub on the contact face. Lock rock, where the blade moves back and forth while locked, is a genuine fault.
A frame lock made of titanium often has a hardened steel insert at the contact face, because titanium galls against steel. See lock bar stabilizer and overtravel stop for the related hardware.