An overtravel stop is a small tab or pin that prevents a liner or frame lock bar from being pushed too far past the blade tang when you disengage it. It stops the lock bar being flexed beyond where it can spring back.
To close a liner or frame lock you push the bar sideways off the tang. Push much further than necessary, particularly on a knife with a short lock bar, and you can stress the spring beyond its elastic range.
The stop gives the bar somewhere to land. Once it contacts, no more inward travel is possible no matter how hard you press.
It is most common on premium folders and on knives designed for hard use. It often takes the form of a small pin through the lock bar, or a shaped tab that meets the blade or the opposite liner.
Do not confuse it with a lock bar stabilizer, which limits outward movement rather than inward.