Please explain amplifier output impedance

I have recently read a few loudspeaker reviews which mention that the speakers would likely work best with low output impedance (or high output impedance) amplifiers.

So, what measurement defines low output impedance (or high output impedance) on an amplifier? What's the numerical value of low and high output impedance, and what is "average"?

Also, what specification of a loudspeaker provides info that would indicate using an amplifier with particular output impedance?

Thanks in advance for explaining this in laymen’s terms. :)