Essentially, a "ground" is an electrical connection to a common return in circuit. In moped terms a ground is a connection to the frame or other metal surface . The frame is connected to the magneto of the moped. Any connection to the frame of the moped is a connection to the return side of the magneto.A magneto is an electrical component upon which the ignition coil and lighting coil are located. The ignition coil provides pulses of electrical power to the spark plug in most mopeds. As no battery or other source of energy is required, the magneto is a self-contained solution to providing ignition and lighting. Power goes from the magneto to the electrical components like the lights or the coil for spark. After power goes to the light it has to return to the magneto usually a wire attached to the frame, this is called the ground. The magneto is also attached to the frame or ground. If the wire does not make a good connection to the frame it's called a bad ground. If you have a battery it's connected to the magneto to charge it, the battery has a negative connection ,that is connected to the frame or ground. A battery with a bad ground will not charge even though the lights will work while the motor is running. The main reason this is done is to greatly simplify the wiring of a moped. Since moped frames are made of metal they provide a perfect ground plane, eliminating the need to run a separate ground wire to each circuit which almost chops the amount of wire necessary in half. Bad grounds can cause poor performance in almost any circuit.