Wtf? First paragraph. No idea. 
Impedance (Z) has many meanings dependending on its application and use, but for simple Audio terms which is what your refering to its the Z input and output stages or devices, ie an Hi level Amplifier driving some speakers will be designed to drive a speaker set up of similar Z (normally in the low Z range), likewise a Pre Amplifier (low level) will have i/p and o/p Z stages (normally a higher Z range).
Impedance is quoted in Ohms normally against a given level (Voltage) but unlike simple Resistance calculation at a DC Level, Impedance is given at an AC level and will change at a different frequency of the AC (Audio) Signal.
Impedance miss match between devices is common today with people that think everything is plug n play now, ie a typical one I see quite a lot is where so called DJ's or Pub Owners will plug the iPhone Head Phone output or speaker out of DAB Radio (both fairly low Z) into a Mixer Desk or PA Input (typically High Z) and then expect it to sound right, while adjusting Volume levels to make amends to get the Volume Level correct, this scenario can have a dramatic effect on Frequency Response, ie, too much or not enough Bass ranging to tinny or distorted Mid to Treble notes, as well as messing up Distortion levels, Signal to Noise ratio, Dynamic Range, Headroom, Ambient Noise compression and overall Music Quality..