The Field Supply Kit (FSK) is an object you can equip on your character via your inventory (I) at a Joker Ammo Vending Machine. Once equipped, press 5 to deploy the kit, your character will crouch down and open a box full of ammunition. Once deployed, your character will automatically begin to resupply ammo and grenade reserves. Other nearby team mates can access the FSK by running up to you and hitting T to resupply. Hit 5 again to pack it up.
As you advance through your starting levels, you will be given a choice between this item or a Mobile Supply Unit, which is another type of supply that turns the trunk of your car in to a supply area. Your car must have an open upgrade slot to mount the supply unit.
Unlocked at Arms Dealer - Grade 6.