Glossary
Pre-fire
In one line: Shooting at a spot before you see the enemy, because you are almost certain they are there.
What it means
Pre-firing goes one step beyond pre-aiming: you do not wait to see the enemy, you start shooting as you come out, at the spot where you expect them. If they are there, your bullets land before they react.
It costs ammo and noise and gives away your position: use it when you have solid information (you heard them, a teammate saw them, it is where they always sit) and when the first shot matters most, like in a clutch or a tight entry.
An example
A teammate saw a defender go behind a box and not come out. You swing already shooting at the edge of the box: the defender was holding slightly off to the side and dies before firing.