Frequency response
How much a device boosts or cuts each frequency — the curve that shows whether it reproduces what it is given or adds opinions of its own.
Feed in a signal of known level, usually a sweep or an impulse, and measure what comes out. The result is plotted as level in dB against frequency on a logarithmic axis. Mathematically the frequency response is the Fourier transform of the impulse response, so either can be computed from the other. Phase response is the other half of the story, the half spec sheets tend to leave out.
"20 Hz–20 kHz" with no tolerance means nothing; it is the "±3 dB" that tells a measurement from a wish. Amplifiers and converters are close to ruler-flat these days. Speakers and headphones are not, and the rooms they sit in even less so — there, reflections and comb filtering shape the curve far more than the electronics do.