Frequency response would be the range of frequencies supported within a specific range (the narrower the range, the "flatter" the response). For example if a system had a frequency response of 20Hz - 20KiloHertz (a Hertz is one cycle / waveform per second) @ .5db that means that within that range, no frequency deviates from the flat power line by more than .5db. If you had two systems that were 20-20K, and one specifies 3db and the other specifies 6db, the 3db system is considered better, because all of the frequencies / sounds are closely matched.
20-20K is specified because that 's the "normal" range of hearing for humans (of course some are better, some are worse, but they gotta use some scale).
Signal-to-noise is the distance between the desired signal (audio in this case) to the noise that is inherent in any system. The "noise" can be hiss, hum, or other interference. High signal-to-noise is desirable. If you were to listen to an input with no signal source, then turn up the volume until you hear a hiss or hum.... that the noise. In a decent system, the noise is so far below the perceptable audio that you don't hear it when listening to something at normal volume levels.
Channel separation indicates how much bleed exists of the signal from one channel (the "left" or "right" side into the other). Signals / music in most sources is multiplexed (blended), Channel Sep shows how well the decoder demultiplexes ("unblends") them. It also may indicate how much crosstalk is happening. Crosstalk can happen when one side leaks or couples to components driving the other side so that left-side output appears to some degree on the right side, or vice versa.
Max output is the highest rated (clean?) signal the amplifier can output.
1 X1/8" stereo minijack is a single place to plug in your headphones, assuming you have headphones with a 1/8" plug (.125")
Harmonic Distortion is one type of many kinds of distrotion that can be introduced to a signal stream as a result of imperfect signal processing and amplification. If you fed an input signal of a perfect sine wave into a system, then compare it to the output waveform, any differences in shape is distortion. Some of the difference in appearance will be due to Harmonic Distortion. HD can be caused by mismatched components, or can be caused by certain efficiencies or inefficiencies in the end-to-end system (input to output). The lower the distortion numbers the better (unless you listen to a lot of Metal ...
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I hope this helps a little ... standard disclaimers apply ... some information bent, folded, spindled, or mutilated for the sake of brevity or simplification.
FWIW
Scott
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