A high-pass filter (HPF) is a EQ filter that let's all high frequencies pass through unaffected, but cuts the low frequencies. For this reason, it is also sometimes referred to as a low cut filter.
On hardware mixers, a high pass filter set to a particular frequency is common. This most often is set at 75 Hz. Some hardware units and most software EQ's, however, will allow you to choose what frequency you want the filter to start at. Some will also allow you to configure the amount the frequency drops off below that point.
These filters are useful for getting rid of low end rumble. A common application is to apply a high pass filter on all instruments in a mix that don't feature bass, thus freeing up the low end for instruments that have do have bass, such as bass guitars and kick drums.
A high-pass filter attenuating frequencies below 75 Hz