A high-pass filter (HPF) passes content above the frequency you set and reduces content below it. It is sometimes called a low cut. When vocals, guitars, or cymbals contain unwanted low-frequency rumble or vibration, it eats into the headroom of the mix. Use it while listening in the context of the song so that you do not remove low end you actually need.
What passes and what is reduced
A high-pass passes everything above the cutoff frequency and gradually attenuates what is below. It does not cut the low end off cleanly at a boundary; the level falls along a slope. The slope is expressed in dB/oct, and steeper settings act more strongly. Both the frequency and the slope decide how it sounds.
Common purposes
Use it to clean up footsteps picked up by the mic, air conditioning, desk vibration, and excess low end from the proximity effect. Tidying pads and sound effects that need no low end makes room for the kick and the bass. Still, the same setting on every track is not the right answer.
Set the cutoff by ear
Raise the frequency until the core or the body of the sound starts to disappear, then come back to where you need it. Something that sounds far too thin in solo can be right in the track. Judging in solo alone, on the other hand, makes you miss low end that overlaps with other instruments.
Watch phase and slope
A steep filter removes unwanted low end, but it can affect the tone and the phase around the cutoff. Compare especially carefully on multi-mic material, on instruments where low end matters, and on the master output. Switch it on and off at matched levels and judge in the context of the whole song.
Connection to EQ
A high-pass is one of the standard filter shapes in an EQ. Clean up unwanted low end first, then shape the tone with a peak or shelf EQ if you need to. The basic rule is to decide your purpose and use it minimally. The role of EQ as a whole is explained in the EQ article.
