How to make single ended voltage measurements
Indeed, if you hold only one probe of your meter to the circuit, then the meter's circuit isn't closed, so there's no voltage difference to be measured.  

Absolute voltage does not exist, it's always relative to some other point. So if you only use one probe of the meter it will say "compared to what?".  

Ground is your reference for all voltages in your circuit. If a voltage is +12 V that will be relative to ground, unless otherwise noted. So you place the meter's reference probe, the black one, on the circuit's ground, and then that same +12 V will also seen by the meter as +12 V, because circuit and meter share the same reference.