The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. The first Ohm's Law calculates the electrical resistance, and its SI unit is the ohm (Ω).
When an electrical current is stablished in a metallic conductor, a high number of electrons can flow freely in this conductor. In this movement, the electrons collide with each other and with the atoms that constitute the metal. Therefore, the electrons have some difficult to move in, in other words, there is a difficulty to pass an electric current through that conductor.