From Halopedia, the Halo wiki
The ballistic assault vest is a body armor system used by UNSC soldiers and Marines. They are worn over the wearer's camouflage utility uniform and are made of hard black nylon.
The vest itself is modular, allowing for pouches for equipment such as ammunition or canteens to be added to it for mission requirements. It also features a holster for a sidearm.
SSgt Avery Johnson and SSgt Nolan Byrne used ballistic assault vests throughout the duration of the First Battle of Harvest, though they are not effective against plasma weaponry. This is because ballistic gear is designed for defense against projectile weapons, such as the ones used by the UNSC.