Here's the scientific explanation of impedance as it relates to speakers. The speaker's impedance is the total resistance, measured in ohms, to the flow of alternating electrical current from the amplifier. Much of that resistance is in the windings of the voice coils in the speaker drivers, but an impedance rating also takes into account any reactive elements in the crossover.
The cables between your speakers and receiver/amp also offer some resistance to the AC audio signal, but it's insignificant (usually about 0.3 ohm or less as long as you use #12 or #14-gauge wire) compared to the speaker voice coils, which typically have a 100 feet or more of narrow-gauge wire in the voice coils.
A speaker's "nominal rated impedance" isn't fixed; it varies with frequency, but the VP150's rating of "6 ohms" means that the VP150 stays above 6 ohms over much of its range, perhaps only touching 6 ohms (or a bit lower) at one frequency. Check the receiver's rear panel to see if there is a switch for "4-ohm" or "8-ohm" speakers and set it according to the receiver owner's manual.
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