Meaning the plant, globe, world is the common reference point for electrical circuits.   It is a large reservoir of "neutral charge" electrical potential and a "zero" voltage reference point for the electricity generation process.

In the generation process, a "neutral charge" at zero volts is taken from the Earth, given energy to form a "energized charge" at very high voltage (thousands of volts). This "energized charge" is then transported via a high voltage distribution system to a substation close to your marina.  The substation reduces the voltage to several hundred volts and then transports the "energized charge" to a transformer located at or near your marina. The transformer reduces the voltage level to 120 volts and sends it to your marina's distribution system. The marina then feeds the low voltage "energized charge" to the dock's shore power pedestal or outlet enclosure located at your slip. 

Your boat's shore power cable connects to the marina's shore power pedestal or outlet and feeds the "energized charge" to the distribution panel on your boat. The boat's distribution panel feeds each electrical device load on your boat. As the device load performs work, it strips the energy from the "energized charge" and returns it to a "neutral charge" (voltage drop). The "neutral charge" at zero volts is dumped onto the "neutral grounded" wire, which feeds the "neutral charge" back to the Earth via your boat's distribution panel, shore power cable, and the marina shore power distribution system to a ground-rod at the marina.