Do you mean impedance the concept (it's just resistance, in the frequency domain), the fact that amplifiers have an "output" impedance, maximum power transfer,

Do you mean impedance the concept (it's just resistance, in the frequency domain), the fact that amplifiers have an "output" impedance, maximum power transfer, or how a transformer works to convert output impedance? There's a lot of inter-layered things behind "why do tube amps need an output transformer." The concept of impedance is pretty simple and doesn't really deal with that, other than being the 0th layer.