What is impedance?

Impedance is the resistance to the electrical current running from the receiver to the speaker.
The lower the impedance, the less work the receiver or amplifier has to do to send more power.

Often, when you read the power specifications of amplifiers and receivers you'll see multiple power ratings in Watts for 4, 6, and 8 Ohms. The power output rating at 4 ohms will be the highest because there is less impedance.

For help selecting the best receiver for your speakers, check out this article: How to match your receiver or amplifier to Fluance Speakers