A Compressor is an audio processor that reduces the dynamic range of a signal. In other words, it decreases the volume difference between the loudest and quietest parts of a sound.

Think of a compressor as an invisible sound engineer with their hand on the fader, automatically lowering the volume whenever the sound gets too loud. This makes the overall audio level more consistent and controlled.

Its operation is defined by four main controls:

1. Threshold: This is the volume "trigger". The compressor only starts to act when the signal exceeds this level.

2. Ratio: Defines how the compressor will reduce the volume. A ratio of 4:1, for example, means that for every 4 dB that the signal exceeds the threshold, only 1 dB will be allowed at the output.

3. Attack: This is the speed at which the compressor starts to act after the signal crosses the threshold.

4. Release: This is the speed at which the compressor stops acting after the signal falls back below the threshold.

Compression is one of the most important tools in mixing and mastering, used to control the dynamics of vocals, give "punch" to the drums, glue the mix together, and increase the overall perceived volume of a song.