Digital audio measures the wave at discrete moments, and a wave needs at least two samples per cycle to be captured at all. Half the sample rate is therefore the ceiling, named for Harry Nyquist of Bell Labs. At 48 kHz, nothing above 24 kHz exists in the file.
Frequencies pushed past the ceiling do not vanish politely; they fold back down as aliasing, false tones with no musical relationship to the source. Hence the anti-aliasing filters in every converter and the sample-rate wars over how much headroom above hearing is worth storing.
For analysis, Nyquist bounds everything: an FFT’s bins run from zero to Nyquist, so sample rate decides the territory the spectrum can even describe.