The stop pin is the small hardened post inside a folding knife that the blade tang contacts to stop it traveling when it opens, and often again when it closes. It sets the blade’s open position and takes the impact every time the knife is flicked open.
Without a stop pin the blade would keep rotating past vertical. The pin gives the tang a hard surface to land against, which fixes exactly where the edge sits relative to the handle and stops the blade over-traveling into the lock.
On most modern folders the same pin also stops the blade when closed, keeping the edge from touching the inside of the handle.
Stop pins wear. Repeated hard opening can flatten a small facet into the pin, which lets the blade sit a fraction further open than designed and shows up as unexpected lock movement or blade play.
Some designs use a hardened, rotatable pin so a worn spot can be turned out of the way. If your lock bar is suddenly sitting further across than it used to, a worn stop pin is one of the first things to check.