Measuring AC Voltage Behind Light Switch
Yes, multimeters measure potential differences: phase to ground, neutral and also phase to phase.
Measuring at a switch, you'd need a hot and a neutral. You can measure to ground if a neutral is not in the box and while you'll get the correct voltage, you can't run a circuit off the two.
Grounds aren't normally floating but neutrals can be. If you have a hot, any ground will work it doesn't have to be from the same circuit.
If there is no neutral or ground at your switch or anywhere near it, you could take an amp reading and then get an accurate resistance reading of the circuit and work it from there: E=RI