Sound filters are processes that allow a certain frequency of sound to be heard while attenuating other sounds. A simple example of this is the bass and treble controls on a typical stereo system. It's commonly known as equalization (EQ), which is really just another word for control.
When considering sound, there are two basic concepts to understand. The first is hertz, which is a unit of frequency that identifies whether a sound is low, mid or high. The second is decibels (dB), which is used to express the relative level of a sound.
Each filter is given a name to designate how it affects frequencies. There are basically two types--shelving and bandpass filters.
Shelving filters allows all of the frequencies below or above its cutoff to pass through while affecting all of the frequencies opposite it. For example, a low-pass shelving filter will allow all the frequencies below its cutoff frequency to pass through unchanged while those above its cutoff will be affected. The inverse is true for a high-pass filter.
Bandpass filters, also known as second-order filters, allow frequencies between two limits or a certain "band." For example, it may allow only frequencies between 40 Hz and 200 Hz. This is controlled by the Q, which narrows or broadens the range of frequencies that are able to pass through, known as the bandwidth. The bandwidth characteristic is 3dB below the maximum center. All other frequencies are blocked.
This is represented graphically on a digital audio workstation via a parametric EQ by bell curves, which can be adjusted by a knob or slider. The outside curves going up on either side are known as roll-off, which occurs at the bandwidth cutoff of 3dB. As with shelving filters, there are high- and low-band pass filters. This type of filter is especially useful when trying to isolate voices or particular sounds, and they are what is used in television detective dramas. In music, they're used more as tone-shaping elements to position a sound in the best place in the spectrum.