Went looking for a definition of "speaker impedance" that didn't require a degree in electrical engineering to understand. The best I found was.
"Impedance:
The load value (in ohms) that the speakers present to the amplifier — the amount of resistance to the flow of current. While playing music, a speaker's actual impedance constantly fluctuates; however, speakers are usually given a single nominal impedance rating for easy comparison. Low-impedance speakers (4 ohms or less) can cause problems with receivers or amplifiers that are not designed to deliver large amounts of current."
