Resistance is a term that is used to quantify how good or bad a given material acts as a conductor. Resistance to flow in a material is quantified or measured in units of Ohms.
Resistance in an electrical system is similar to the friction or head loss in a water pipe that causes the water pressure to drop as it moves from one end of the pipe to the other. The more resistance that must be overcome in the wire, the more pressure or voltage loss, commonly called line loss, found in the system. This is important because if too much voltage is lost, equipment on the far end of the system may not operate correctly.
Once again, there are several symbols for resistance, or Ohms: an R which is intuitive, and the Greek symbol Omega (Ω).
In this graphic you can see that resistance is impacted by several different qualities of the wire through which electrons are flowing. A primary factor is the length of the wire. The longer the wire, the more resistance it carries and the more line loss that will occur. The material chosen as the conductor is also an important factor. Since copper is a better conductor than aluminum, it will have a lower resistance, and thus a lower value of Ohms and line loss. The diameter of the wire is also an important factor. The larger the diameter, the less resistance and line loss it will have.