The high frequency limit of hearing is the upper extent to which a particular animal can perceive sound.
Perhaps the most commonly known aspect of the psychoacoustic model is that humans cannot hear frequencies above and below certain thresholds; in fact, most humans can only hear frequencies between 20 Hz and 20.5 kHz. So-called "silent" dog whistles exploit this phenomenon by producing sounds at frequencies higher than those audible to humans but well within the range of a dog's hearing.
Likewise, when compressing a digital signal, an acoustic engineer can safely assume that any frequency beyond approximately 20.5 kHz will not have any effect on the perceived sound of the finished product, and thus use a band-pass filter to cut everything outside this range. The sound can then be sampled at the standard CD sample rate of 44.1 kHz, set somewhat higher than the calculated Nyquist-Shannon rate of 41 kHz to allow for the cut-off slope of a reasonable band-pass filter.