Noise reducers are filters that remove certain frequencies or ranges of frequencies from digital audio recordings. Most video editing programs, even low-budget ones such as iMovie, will generally include a few basic noise reducers for editors to use. Most are designed to deal with specific problems you will run into in the field, such as humming noises, or vocal problems, such as sibilance. The best can analyze a section of noise and remove the frequencies in it.
Noise reducers should be used only in conjunction with other audio editing techniques, and preferably should be the last stop in your audio workflow. This is because noise reducers may remove wanted audio, such as room ambiance, and may create other problems, such as artifacts or distortion. Unwanted sounds should be edited out, removed with equalization, or mixed down. If the sound cannot be mixed out, then you should employ noise reduction.
All noise reducers will have three standard controls: attack, release and gain. Attack and release engage when the reducer hits a certain frequency. Attack is the time frame that the reducer is used, and release is how long the filter will last before shutting off. Gain is how strong the signal will be when it leaves the filter. You should experiment with all three in order to find the best settings for any noise reducer.
There are several common types of noise reducer. "Hum" and "buzz" reducers remove a specific frequency, usually in a high frequency range. "Pop" and "click" filters are designed to remove short, sudden sounds that may occur on older audio. "Broadband" filters will remove a wide range of frequencies with different attack and release for each. "Comb" filters remove a few specific individual frequencies. You will often need to use several filters together to remove unwanted audio.
Noise reducers cannot create audio; they can only remove it. Noise reduction will not make badly recorded audio good. Too much noise reduction can remove wanted audio in the mix, such as reverberation from a live music venue or room tone. It may also make certain audio sound muddled, as if it was recorded underwater. Finally, the computer, after removing frequencies, may play back the remaining audio with odd effects called "artifacts," which are created by the computer trying to "fill in the blanks."