Also known as a high pass filter. Basically this is a type of filter that removes low frequencies from an audio signal. Normally they are designed so they remove frequencies below a certain determined frequency (often somewhere between 20 Hz and 150 Hz). In typical designs these filters have slopes, which means there is more and more attenuation as the frequency gets lower. So right around the rolloff (or cutoff) frequency the signal may only be down 3dB to 6dB (3dB is standard), but depending upon the design of the filter, lower frequencies may be considerably more attenuated. This is usually rated in dB/octave, or decibels per octave of rolloff. If your filter is at 150 Hz it is safe to assume the signal will only be reduced by 3dB at that frequency. However, one octave below that, at 75 Hz, your signal may be attenuated 15dB, or 12dB more. This would represent a 12dB per octave rolloff, which is common.