What is resistance?

Resistance is the property of a material to oppose the flow of electrons through it (current). When electrons flow through a material, they collide with the other atoms in the material and lose its energy. The loss of energy results in the drop of potential energy across the material. 

R = (V/I)

V = potential difference across the material.
I   = current flowing through the material

The units (measurement) of resistance are referred to as ohms.