Propellant depot

Propellant depots are like very big "stores" for fuel, so that spacecraft can top up their tanks and get the fuel they need. Picture a gas station, but in space. Instead of filling up your car with gasoline, a propellant depot fills up a spaceship with fuel. They help spacecraft travel further and longer because when a spacecraft needs fuel, it doesn't have to turn back or land somewhere - it can just head over to the depot and get more fuel!