From bottom to top:
The thick line is your main tank Thats where your Frameshift-jump fuel comes from.
The thin line above that is the so called "reservoir", a small tank, (0.5 tons on the sidey, 1 ton on any other ship) which feeds the Powerplant and the thrusters.
It will be refilled automatically from the main tank whenever it runs empty.
The number above that is your momentary fuel usage.