The earth is the zero-volt reference for power distribution systems. In North America and other countries it’s called ground, and in Europe, Australia, and other countries it is called earth.
Voltage is only meaningful when it is referenced to another point. When a bird lands on a high-voltage wire, it doesn’t get electrocuted because it does not complete a circuit to a zero-voltage reference or to another point in the circuit with a different potential.
If the bird happens to straddle the gap between the high-voltage line and the metal transmission tower or another line, sparks will fly. That’s because the voltage needs a reference.
We typically take zero volts as the absolute reference for voltage measurement. The exception is when we want to know the voltage drop across a particular component such as a resistor or a transistor.
But normally, for example, a 12-volt DC power supply means that the positive terminal is 12 volts higher than a zero-volt reference. When we say a voltage rail is at 5 volts, that implies that is has been referenced against zero volts and it is 5 volts below the reference.
Without some reference point with which to compare, voltage measurements are meaningless. But what is our zero-volt reference based on?
The zero-volt reference is the earth, the largest current sink available to us. The earth is actually a conductor, although various parts of it are better conductors than others. Soil composition, moisture content, mineral content, and other factors influence the impedance of the soil at any given location.
But the earth is a very large current sink, and as long as we can establish good contact with it, we have a good zero-volt reference. Every power distribution system has at least one point that is electrically connected to the earth, usually by means of a copper rod driven into the ground.