A voltmeter measures the electrical potential difference between two points in an electric circuit.
What is the relationship between voltage and current?
Ohm's law applies to electrical circuits; it states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.
Or simply V = I*R
Note that Ohm's law apply only to ohmic materials i.e. Materials which obey ohm's law.
Not all materials follow this V = I*R relationship.