If you want to measure the height of something, you need to stand somewhere and call that "zero." There is such a thing as absolute sea level, H=0, and electrical earth ground, V=0, but they may not always be available.
So we clip the "negative" (better called the "reference") wire of a voltmeter to something conductive and declare that to be "ground", namely zero volts. Chassis, car body, metal ladder that we're standing on, most anything. Then we measure positive or negative voltages relative to that.
We have one Tek oscilloscope with fully isolated channels. We can clip a scope probe ground to the hot side of the AC line and then probe relative to that.