Phil is correct. Impedance is the "load" presented to the amplifier via the Loudspeakers.
Here is an old analogy that was presented to me about 35 years ago and it holds true today. Bear with me on this because it does involves complication in a rather easy to understand setting.
Think of the circuit inside the receiver (called the amplifier) as the "motion force". Now think of the loudspeaker elements (that move back and forth) as a pulley and rope system. Think of impedance as a "brake" on the pulleys. The lower the impedance, the more force it takes to pull the rope because of more "braking action" presented by the pulleys.
Loudspeakers have a "nominal" impedance which hovers in the area of the rating. A typical 8 ohm loudspeaker system can drop to well below 8 ohms and rise well above 8 ohms. As different ranges or "frequencies" are presented to the loudspeakers, the impedance changes. Some loudspeakers change little in impedance and some loudspeakers change quite a bit. This "impedance curve" of the loudspeaker system is a result of the variances in capacitance, inductance and resistance and other forces.
An 8 ohm speaker can actually drop at times to below 4 ohms or rise to as high as 50 ohms depending on the frequency (in cycles per second) presented to it or a combination of multiple frequencies as in MUSIC.
So the moral of this story is most receivers that are rated to DRIVE a nominal 8 ohm load usually have no real problem driving a 4 ohm load BUT....because the 4 ohm load can vary down well below 4 ohms in a 4 ohm loudspeaker, protective circuitry in the receiver may activate if the tempurture rises too high in the amplifier circuit of the receiver.
Now, back to the analogy. Think of the pulley brakes. Excessive braking will result in escessive HEAT. When your receiver is presented with a nominal 4 ohm load, it will run hotter, especially at higher volume. This in turn may activate the protection circuitry inside the receiver to protect its amplifier from escessive heat generated by the extra FORCE demanded by the loudspeakers.
Rule of thumb is if a receiver is deemed proper for an 8 ohm load, their is usually enough saftey margin to drive a 4 ohm load. Going below 2 ohms continuously may cause burnout and damage.
I know this sounds confusing but the analogy should prove useful in explaining that the Loudspeaker is the LOAD and the receiver is DRIVING that load. Lower impedances are harder to drive than higher impedances and draw more power from the receiver or amplifier.
When connecting multiple loudspeakers to a receiver ALWAYS follow the manufactures guidelines.
