🚀 go-pugleaf

The purpose is safety of humans (and cats, etc.).
A 120V branch in a modern US residence is wired...

  Function  Wire Color  Plug or Socket Shape
  --------  ----------  --------------------
  Hot       Black       small rectangle
  Neutral   White       large rectangle
  Ground    Green       round

The neutral and ground wires should be tied together at
the entrance box and both earthed.

The hot wire varies in voltage sinusoidally relative to
the neutral wire.

Equipment sees a voltage difference between the black and white
wires and it does its work.  Whether your reference is black or
white means only a 180 degree phase difference.

Relative to a cold water pipe or a puddle of water on the floor
there is a big difference.  A human simultaneously touching
a pipe and a white (neutral) wire should experience no shock.
Touching the black (hot) wire is something else. A 120V, 10A,
low-impedance source is often fatal.

Consider a lamp.  If the house and lamp are correctly wired,
the center button of the screw socket for the lamp should be
hot and the shell neutral.  Then someone changing a bulb with
the switch on is less likely to touch a "hot" part.

Checking house wiring...

  Voltmeter   Point A           Point B           Reading
              -------           -------           -------
              small rectangle   large rectangle   120 vac
              small rectangle   round hole        120 vac
              large rectangle   round hole        0

  Only after passing the voltage test above, you can check with
  an ohmmeter that the large rectangle and the round hole show
  very low resistance between them and each of these to earth
  (a continuous copper cold water pipe, for example).

For a lamp, check for continuity between the shell and the large
rectangular spade on the plug and between the button and the small
spade.