In order to understand this you have to think about what each meter is measuring. I'll assume you know and understand what the ammeter is measuring, so we won't go over that.
The voltmeter, of course, measures voltage. If you remember back to your first physics class, voltage was defined as a potential difference between two points in space. Notice the word "difference." This implies that you need two distinct points in space at two distinct potentials in order to measure a potential difference between them, or a voltage. Voltages are never absolute, they are always measured with respect to something. If you were to hook up a voltmeter in series with a circuit component, you would read zero. This is because you've essentially connected your voltmeter in parallel to one point in the circuit. There can be no potential difference if there is no difference in space.
I hope that helps.