The fuel tanks store the hydrogen collected by your ship's intakes and deliver it to the power plant, which in turn delivers energy to supercharge the fuel and expel it at speed. If you run out far from civilized space, you better hope you have triple A.
Bingo Fuel Edit
Bingo fuel is a the critical amount of fuel, below which you will not be able to reach a friendly base. Your ship computer will warn you before reaching bingo fuel.