Fuel allows your ship to travel between derelicts and systems. Your ship starts with 6 fuel in the main tank, and all fuel collected goes into the reserve tank. Fuel is collected from derelicts by gathering the fuel with a drone at the fuel access point.
There are two types of fuel in the game, P-Fuel (Propulsion fuel) and J-fuel (Jump fuel). P-Fuel is used to travel inside a system, and J-Fuel is used to travel between systems. Traveling outside systems requires a Stargate. Whenever J-fuel is expended to travel to a new system, the main tank is refilled with P-fuel. The reserve tank is unchanged.