In very simple terms, compressors are like automatic volume controls that kick in when the input signal exceeds a preset level. Then, once the input level has fallen back below that threshold level, the compressor resets itself and ceases to have any effect on the sound.
Depending on the sophistication or design of the compressor itself, users may be able to set the level at which the compressor is activated, the speed of response, the speed at which it resets, and the overall output level of the compressor itself.
