A compressor automatically reduces the volume of audio that exceeds a set threshold. This narrows the dynamic range — the difference between the quietest and loudest moments — making the signal more consistent.
Key compressor parameters: threshold (the level where compression starts), ratio (how much the signal is reduced), attack (how fast compression engages), release (how fast it lets go), and makeup gain (to restore lost volume).
Compression is used on virtually every track in modern music: taming vocal dynamics, adding punch to drums, glueing a mix together on the master bus. Over-compression strips life and dynamics from music — subtlety is key.