From MopedWiki

A petcock (fuel on/off switch) is used to control the flow of gas to the engine and is located at the bottom of the gas tank. The petcock usually has three settings: On, Off, and Reserve. Normally the petcock is in the on position during use but when fuel gets to a low level, then you would be required to switch the petcock to the Reserve position, allowing fuel that is lower in the tank to pass through the petcock.