How it works
Ground (often abbreviated as GND) is the reference point for all voltage measurements in a circuit. Voltage is a measure of potential difference between two points, so we arbitrarily define the Ground point as exactly 0 Volts. All other voltages in the circuit are measured relative to this point.
In portable electronics, Ground is simply the negative terminal of the battery. In mains-powered systems, Earth Ground is physically connected to the earth via a metal rod, providing a safe path for fault currents to dissipate to prevent electric shock.