A portable ballistics shield similar to those in use by various Police SWAT teams around the world. If the player gets close enough to the enemy, the riot shield may be used as an offensive weapon by being able to bash the enemy with said shield.
In Campaign you can pick these up (They appear on your GPS if you can't find one) and have your teammate shoot from behind it. Just tell him to regroup and he'll come to it.
To use a riot shield, the player can either go up to the car it is attached to and break it off or they can shoot it off, which is quicker but there is obviously going to be ammo consumption.