You have two fuel tanks. One main tank and one reserve tank. The main tank is where fuel is taken from when you jump. Above the main tank gauge is the reserve tank gauge along with an indicator of your current fuel usage. When the reserve tank is emptied, it gets refilled from the main tank and the main tank goes down. In ships where the reserve tank is a significant portion of the main tank, the main tank gauge will be divided into sections of the size of the reserve tank. When the main tank is drained you can no longer make any jumps. When the reserve tank is drained, your modules will not get power - including life support, and you will die from lack of oxygen unless you are refilled before the oxygen timer depletes.