The HPF is either a small stand-alone unit (preferably either the FDeck by Francis Deck in the States or the Thumpinator by Max Greco of [sfx] here in the UK), or a feature of your existing DB preamp, e.g. the 'Depth' knob on a Fishman Plat Pro.
It's useful for (i) taming feedback, if you suffer from that, and (ii) reducing speaker excursion at very low frequencies.