Frequency response is the range of audio frequencies (in Hz) a headphone or speaker can reproduce, typically given as a low-to-high range like 20Hz – 20kHz with a tolerance.
Human hearing: 20 Hz to 20 kHz at birth, narrowing to ~16 kHz by adulthood. Bass: 20–250 Hz (felt as well as heard). Mids: 250 Hz–4 kHz (vocals, most instruments). Highs: 4 kHz–20 kHz (cymbals, sibilance, air).
The stated range only tells half the story — accuracy across the range matters more. Look for FR graphs (RTINGS, Crinacle) showing how flat or shaped the headphone is. Most consumer headphones boost bass and treble (V-shape) for excitement.