It selects what frequency band will play. So you would have low (low pass filter, lpf) on your subwoofers so they only played low tones. HPF is high frequencies for mid and tweeters, or you can band pass for a mid with just middle frequencies, depending on how you are running a system. There are active types that go inside or before an amp or passive that go on the speaker. Active can be changed easily and your amp does not waste power making sounds that can't be played.
