All electrical power needs 2 wires. Kinda like 'supply' & 'return' for the electricity.

The supply line comes from the (+) terminal of the battery. First it goes thru fuses & switches & whatever...

The 'return' wire actually goes to some handy place where it's screwed down to the metal body of the car. Sometimes it's not even a wire; it's just grounded thru the mounting bracket. THAT'S called grounding. ALL the electricity flows back to the (-) battery terminal thru the car body.

Sometimes when people talk about grounding something, what they really mean is replace the existing ground wires with some bigger wires or better connectors for the ground wires.