A slide stop, also referred to as a slide release, on a semi-automatic handgun is a function that visually indicates when a handgun has expended all loaded ammunition

Ammunition

Ammunition is a generic term derived from the French language la munition which embraced all material used for war , but which in time came to refer specifically to gunpowder and artillery. The collective term for all types of ammunition is munitions...

 and facilitates faster re-loading by removing the step of pulling back the slide

Pistol slide

The slide is the part of the weapon on a majority of semi-automatic pistols that moves during the operating cycle and generally houses the firing pin or striker and the extractor, and serves as the bolt...

to advance the first round of a new magazine. In many cases, this is achieved by replacing the magazine and activating a push-button or lever release to load the first cartridge into the chamber.

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 bas 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.