From Halopedia, the Halo wiki

Spread refers to a gameplay system common to first-person shooters, which causes the projectiles fired by a weapon to deviate from the direction specified by the reticle. One frequent form of spread is random spread, where this deviation is chosen at random, within certain bounds. Random spread is commonly used in all Halo first-person shooter games, particularly for automatic weapons, to limit their usefulness at long ranges.

In some games, certain weapons' spread radius increases as the weapon is fired, a phenomenon known as bloom.