Glossary
Pre-aim
In one line: Placing your crosshair where an enemy probably is before you see them, as you come out of cover.
What it means
Pre-aiming is crosshair placement applied to a specific angle: you know defenders often sit in that spot, so you come out with your crosshair already on their head. If someone is there, you only have to shoot.
You need two things to do it well: knowing the map's common positions and moving so you uncover one angle at a time. A player who pre-aims every common spot while entering clears a site in seconds.
An example
You walk into a site where defenders often sit behind a box. As you leave the entrance, your crosshair is already on the edge of the box at head height. The defender is there: one shot before they react.