A slide stop, sometimes referred to as a slide lock or slide release, on a semi-automatic handgun is a function that visually indicates when a handgun has expended all loaded ammunition and facilitates faster reloading by pulling back the slide or depressing the slide lock to advance the first round of a new magazine.
The various terms relate to the two functions of the component: while it automatically catches the slide (locking it back) after the magazine's last round has been fired, thereby allowing the user to easily release the slide by pulling down on the switch, it also allows the user to purposefully stop or lock the slide back by pressing up on the switch while racking the slide. Some manufactures recommend using the slide lock as a release, others recommend racking the slide. Using the slide lock as a release can accelerate wear in some models. Check with your manufacturer for proper operation of individual pistols.
The slide stop has a protruding part through a hole in the gun frame, a hole in the magazine, and a level to lock. When the gun is loaded, the cartridge or the bullet size, shape is small enough to not contact the protruding part. When the magazine is empty, the feeder pushes the protruding part up and the slide stop level moves up by the magazine spring power until it reach the highest position as the carved part of the slide, stopping the slide from being pushed forward. The user can push the slide stop down to release the slide or pull the slide backward, the slope contact of the slide will push the slide stop down indirectly. Almost all semiautomatic handguns have the slide stop in a form of a level switch.