An intentionally grounded wire that completes a circuit should a neutral wire fail. Typically a green wire. Electrical current takes the shortest, most direct path to ground. In your electrical circuits, the black wire (hot) supplies power and the white wire (neutral) carries the current back. If the neutral wire was somehow broken or interrupted, YOU would be electricity's shortest path back to the earth, so the ground wire is like an insurance policy to carry the current back so you don't get shocked in case the neutral wire fails.