Fuel was introduced as a mechanic in Battlefield V specifically for the Firestorm gamemode. In Firestorm, all ground and air vehicles have a limited amount of fuel that depletes when the vehicle is occupied. When all fuel is expended the vehicle may no longer move.
Fuel Canisters can found in the game world and can be used to replenish the fuel bar of vehicles allowing them to be used longer. Once obtained, players can equip and resupply vehicles. During a resupply, the contents of the canister will slowly drain to zero.