Glossary
Spray
In one line: Holding down the fire button and shooting many bullets in a row while controlling recoil.
What it means
A spray is the long burst: hold the trigger and the gun keeps firing until you let go. The first bullets go where you aim; then recoil pushes them up and sideways, in a pattern that is always the same for each gun. Controlling a spray means pulling your mouse down to compensate.
It is mostly for close range, or when several enemies appear together. At mid and long range a burst or a tap is almost always more accurate.
An example
Two enemies come out of a corner together a few meters away. No time for single shots: you hold the trigger on the Phantom, pull the mouse down slightly and move from the first to the second. Two kills in one spray.