Spread is an abstraction of various real-life conditions which can cause a weapon's round to not go where the perceived aimpoint is. When a weapon is fired, there is essentially a random chance for it to go in any direction within a small area, called the cone of fire. When the weapon is fired, the game engine may apply a slight variance in a particular direction, after which the projectile will only be affected by gravity.
On many automatic-fire weapons, spread will increase the longer the trigger is depressed, making later shots less likely to hit their intended target. Spread may also be wider when firing from the hip than when aiming down sight, and may increase when the player is suppressed. Spread may be reduced while crouched or prone, although it may briefly increase while in the act of crouching or going prone.
In some games, light machine guns may start with a large amount of spread that decreases over time, making LMGs more valuable for suppression and situations where they can maintain sustained fire. Sniper rifles will often have zero spread while aiming down sight, and wild spread when hip-firing. Submachine guns and Personal defense weapons will typically have tight spread when hip-firing, potentially eliminating the delay of aiming down sight.
Weapon attachments may alter how much spread a weapon has, or the rate at which spread recovers over time.