I'll keep it sweet and simple.
The ohm rating of a speaker is called it's impedance.
The impedance of a speaker is no indicator of how good or bad it sounds.
A speaker rated at a lower impedance will place more "stress" on an amplifier than a speaker of a higher impedance.
The ohm rating of a speaker is an approximation since it varies with the frequency. Generally, the lower the frequency, the more likelyhood of impedance dips. Sometimes, these dips can be significant.
