Electrical resistance (or the resistance to the flow of electricity through a material) is measured in units called ohms (Ω). You can think of resistance as a measurement of how loosely or tightly a material holds on to its electrons. Some metallic materials like gold, silver, copper, and aluminum have low resistances to the flow of electricity and make good conductors so they are said to have a low resistance to the flow of electricity. Other materials like porcelain, rubber, plastic, and glass have a very high resistance to the flow of electricity and do not allow electricity to flow through them. The lower the resistance of a material (measured in ohms), the better the material acts as a conductor.
Consider a water piping system as an analogy to an electrical system. Several things can resist the flow of water in the pipe. These might include friction caused by rust or corrosion, objects stuck inside the pipe, or even the size of the pipe itself. The same is true of electricity. Smaller wires have more resistance than larger diameter wires because they contain less atoms from which electrons can flow, while longer wires have more resistance than shorter wires because electrons are required to move longer distances.
When electricity flows through any resistance, energy is dissipated in the form of heat. If the heat becomes intense enough the conductor resistor may actually glow. A good example is an incandescent light bulb. When electricity is run through it, the filament resists the flow of electrical current. This resistance causes the filament to heat up and glow, which in turn produces light.
The scientific symbol for electrical resistance, measured in ohms, is the Greek Omega Ω. However, electricians and practical wiring books often use an "R" to represent resistance. This can create some confusion since either may be used.