In motorcycles and cars, fuel reserve is the extra fuel reserved by the manufacturer in the fuel tank before the main fuel capacity runs out.
In motorcycles, when the main fuel runs out, the rider has to manually turn the reserve knob, usually located in the left part of the engine, to the 'reserve position'. This allows the rider to use the remaining fuel.
The purpose of the fuel reserve is to serve as a warning to the rider that fuel is low, as many motorcycles do not have fuel gauges and determining the fuel remaining is difficult. When the main fuel is exhausted, the motor will stop running, prompting the rider to change the position knob to continue riding with a known small quantity of fuel. Often when a rider notices the motor start to sputter, he or she can access the reserve before the engine shuts down.
In most cases, the main and the reserve settings are actually drawing from the same tank, but there are two outlets through which the fuel may leave. One outlet is located a short distance above the other, when the fuel selector is set to the 'main' position, the fuel will flow from the upper outlet, and will stop flowing when the fuel level gets below the outlet. When the selector is on 'reserve', the lower outlet will be used, which allows all the fuel to be drawn from the tank.
Because in most cases the 'reserve' setting simply allows access to all the fuel, rather than a dedicated tank, the 'reserve' selection allows the rider to deplete the main fuel supply as well as the reserve. This causes no harm, except that the fuel may run out without warning.