Controlling the dynamic range. Ever have your hand on the volume knob while listening to a speech to make sure you hear the quiet parts then ducking down when it gets loud. We can’t have our vocal or instrumental tracks varying in volume like that so we compress them to even them out.
Once we do that, we have a noticeable drop in volume, so we will need to boost the gain to make the vocal sit where it was in the mix.
Threshold: This is the level of volume at which the compressor starts controlling the volume (compressing)
Ratio: How rapidly the signal gets compressed. It’s a fraction and the changes here can be dramatic quite quickly. The higher the ratio, the faster the sound is being shrunk to keep it in line.
Output: The overall gain addition now that you have shrunk the sound. Makeup Gain.