A kilohertz (kHz)1 is 1000 hertz (Hz). This unit of measurement is used quite frequently by humans as human hearing ranges from 20 Hz to 20 kHz, and human speech ranges from 100 Hz to 17 kHz,2 and most of the acoustic information in that speech falls in the 3–5 kHz range.3 Hearing screens and audiograms generally measure from 250-500 Hz at the low end and 4-8 kHz at the high end, with 500, 1000, 2000, and 4000 being the most commonly tested frequencies.
Footnotes:
1. Hz and kHz have the weird capitalization because the hertz is named after physicist Heinrich Hertz; while the hertz and kilohertz do not maintain the capital letter, the abbreviations do.
2. While different sources vary a bit on the exact ranges, and differences between individuals can be quite large, it is common to split these values between male and female speakers: female vocal range during normal speech ranges from about 350 Hz to 17 kHz, while male vocal range during normal speech ranges from about 100 Hz to 8 kHz.
3. Acoustic information refers to the phonological features that give us the meaning in speech, most particularly, the features that allow us to distinguish one speech sound (a vowel or consonant) from another.