After putting a loaded magazine in the magazine well, you let the slide slam shut to push a round out of the magazine and into the chamber. At the same time the rear edge of the trigger bar ‘grabs’ the firing pin lug
and pulls the firing pin part of the way back to a ‘partially cocked’ position.
A lot of Glock owners (and some ‘would be’ Glock owners), are afraid that the trigger spring on their Glock will break and then the gun won’t work. Not true! You just need to know how to work it!
Some Glockers will go so far as to replace the coil trigger spring with a New York trigger spring to insure that they don’t have a breakage. This does work OK, but it also makes the trigger pull a whole lot heavier, and it’s really not needed.