It's simply the highest voltage at the largest current the amplifier can drive without distorting the signal by a set amount usually measured as total harmonic distortion (THD).
You should know the formula V= IR .... For AC signals Resistance R becomes Impedance Z and so
V= IZ or as we want Z ... Z = V/I
So the smallest impedance an amplifier can drive is the largest undistorted voltage (Vmax) divided by largest undistorted current (Imax).
So Zmin = Vmax/Imax
It is obvious on sound systems as the Impedance of the speaker is fixed the only way to make it louder is to increase the voltage output. At some point that will lead to the situation the amplifier output current reaches a point it starts to distort or the waveform reaches or the output voltage that exceeds the peak to peak rating. Whichever happens first creates the minimum impedance the amplifier can drive.
Speakers are generally design around certain fixed impedance standards 8,16 etc. So generally the louder you want them the higher the output amplifier voltage has to go which is done by varying the gain somewhere along the amplifier stage. That should give you enough to understand.
Works the same on Digital IO .. if an I/O pin can drive 10ma at 3.3V
ZMin = 3.3V/0.01A = 330 Ohms.
So that IO pin can't drive a load lower than 330 ohms.
post edited by LdB_ECM - 2020/05/15 08:07:12