The Nyquist-Shannon sampling theorem is the rule that makes digital audio possible: to represent a signal perfectly you must sample it at more than twice its highest frequency. The half-the-sample-rate limit is the Nyquist frequency, so 48 kHz sampling covers everything up to 24 kHz, just beyond human hearing. Anything above that limit must be removed by an anti-aliasing filter before the ADC, because frequencies past Nyquist don't disappear — they fold back into the audible band as false tones (aliasing). The theorem is why 48 kHz is enough for video, and why pushing higher mainly buys filter headroom rather than audible bandwidth.
