A lock bar stabilizer is a small washer or bushing fitted over the outside of a frame lock bar to physically stop it from being bent too far outward. It prevents the most common way an owner ruins a frame lock.
A frame lock bar is a spring. Push it too far out, whether by over-enthusiastic closing or by dropping the knife, and it can take a permanent set, after which it no longer presses firmly against the tang and lockup goes soft or fails.
The stabilizer sits proud of the lock bar and hits the surrounding frame before the bar reaches its elastic limit. It is a hard stop, not a spring.
It is not an overtravel stop, though the two are often confused and sometimes combined. A stabilizer limits how far the bar moves outward, away from the blade. An overtravel stop limits how far it moves inward, past the tang.
Not every good frame lock has one. Plenty of well-made knives simply use lock geometry that cannot be over-flexed in normal handling.