The H3PO4 acid puts the hydrogen ions in solution. Also make sure that the scale is correct. All multimeters are different, but there should be a low scale, something like 2V, or 6V. Attach wires from the meter's probes to the metal strips. Remember that in a battery, one metal strip is becoming positive and the other negative. Voltage is a measure of this charge difference.
Voltage is a lot like a hill.
A hill is due to a height difference. Voltage is due to a charge difference. Say you roll a ball down a hill. The higher the hill, the faster the ball will go. A ball rolling down a high hill has more energy than a ball rolling down a low hill.