The resistance to the flow of an electric current in a circuit measured in ohms. Impedance is what restrains the flow an electric current making it more difficult for the current to move through a circuit. Impedance is similar to a dam in a river. If the river is a circuit (to be a true circuit the river would need to run in a complete circle) then the water is the electric current. The water flows freely when there are no obstructions to resist the flow. The dam in the river resists the flow of water. By fully opening the dam there is no resistance. The further closed the dam becomes, the more difficult it is for the water to flow.
A dam of 1 ohm would have very little resistance (although a dam's resistance to water is not measured in ohms, impedance is and so ohms will be used for this example). As the dam's resistance to the water's flow, it's impedance, increases to 2 ohms it becomes more difficult for the water to flow. Similarly, as the dam's impedance grows to 4 ohms and then 8 ohms and beyond it becomes more and more difficult for the water to flow. In an electric current, low impedance lets more current flow.
A well designed amplifier with a strong power supply will double the amount of power in watts that it sends to a speaker with each halving of impedance. For instance, an amplifier might send 100 watts to a speaker with an 8-ohm impedance. When the impedance is halved to 4 ohms that amplifier would send out 200 watts of power. In the real world, few amplifiers are actually able to double their power output as impedance halves due to limitations on their power supply and design. Additionally, few amplifiers are able to put out power for any significant length of time with impedances of 3 ohms or less (the amplifier must be able to supply huge amounts of power as the impedance decreases below 4 ohms - in fact, many receivers cannot operate below 6 or even 8 ohms).
When shopping for speakers, you will see figures stating the nominal impedance of a speaker (most speakers have a nominal impedance of between 4 and 8 ohms). This is the speaker's average resistance to the flow of current through the circuit going from the amplifier into the speaker and back to the amplifier. However, this figure is only the average impedance rating across all frequencies into which the speaker produces output. In reality, the impedance may drop at some points to 2 or 3 ohms and may reach 40, 50 or even 60 ohms at other frequencies. For this reason, it is beneficial to have a strong amplifier able to produce large amounts of power down into small impedance loads.
Generally, receivers should be mated with speakers having a nominal impedance of 8 ohms or so while separate amplifiers may be used with speakers with nominal impedances of 4 ohms or so. A speaker with an 8 ohm nominal impedance will not generally have as low a minimum impedance as a speaker with a lower nominal impedance such as a 4 ohm speaker. The most important impedance rating to be aware of is that used with speakers. You may also see impedance ratings for speaker wire and other types of wire. In most cases thicker, heavier gauge wire (such as 10 or 12 gauge) will have a lower impedance and allow more current to flow through. In essence, lower impedance means lower resistance allowing more power to pass through while higher impedance restricts power thus restricting the amount of current able to flow in a circuit.
