Threshold
Definition
The level at which a dynamics processor begins to act. Signals below the threshold are unaffected; signals above it are processed according to the ratio, attack, and release settings.
In Simple Terms
The volume level where a compressor, limiter, or gate starts working. Anything below the threshold passes through untouched. Anything above it gets compressed. Lower the threshold to catch more of the signal.
In Practice
A compressor with a threshold of -20 dBFS begins reducing gain whenever the vocal exceeds this level, controlling the loudest moments of the performance.