A high-pass filter passes highs and removes lows: set the cutoff at 100 Hz and rumble, footsteps, and mic handling below it fade away while the material above continues untouched. The other name, low-cut, describes the same filter by what it removes, and mixers print it on the channel strip as a small slashed curve.
It is the most-used filter in audio because low-frequency junk accumulates everywhere: stage rumble, air conditioning, plosives, the mud of many instruments overlapping below 200 Hz. Engineers high-pass nearly every non-bass channel out of habit, leaving the low range to the kick and bass that own it. The cutoff’s steepness is the slope, quoted in dB per octave; 12 and 24 are common.
In audio analysis the same filter isolates what a detector should care about. Splitting a signal into bands with high-pass, low-pass, and band-pass filters is how a visualizer hears bass, mids, and air separately, which is the first step of every frequency-band reactive design.