some guns (Taurus and some S&W) have a built in lock the come with a key (sometimes it looks like a allen wrench) turning this will deactivate the firearm (normally it deactivates the trigger or locks the hammer) not many guns are putting internal locks on their guns and a lot that did (ruger Blackhawks) have stopped doing it

It is an internal locking mechanism that locks the "Action" from functioning and renders toe firearm completely inoperable. It is usually a key that lock/unlocks the mechanism. Example of a semi auto internal lock here : thefirearmblog.com/blog/wp-content/uploads/2014/05/TFB-Taurus-Millenium-G2-Review-02.jpg