Many materials contain some 'free' electrons which can move in response to an applied electric field.
By attaching a pair of metal wires and applying a voltage between them we can move these charge
carriers through the material. The result is a current whose magnitude depends on the characteristics
of the piece of material and the applied voltage.
Ohm's law says that the current we get is proportional
to the potential difference between the wires. Most common conducting materials obey Ohm's law.