In simple terms, a compressor is an automatic volume control. Loud sounds over a certain threshold are reduced in level; quiet sounds are not reduced. In this way it reduces the dynamic range of an audio signal.
A compressor reduces the gain (level) of an audio signal if its amplitude exceeds a threshold. The amount of gain reduction is determined by a ratio.
For example, with a ratio of 4:1, when the (time averaged) input level is 4 dB over the threshold, the output signal level will be 1dB over the threshold. The gain (level) has been reduced by 3 dB.
When the input level is 8 dB above the threshold, the output level will be 2dB; a 6 dB gain reduction.
A more specific example for a 4:1 ratio:
Audio input gain
Audio output gain
Sets how long it takes for the compressor to switch from reduction to a new ready state (release)
Sets the amount of process.In a normal use, if the Ratio is 3:1, signals 6db above the threshold will come out 2db over the threshold.
Sets the level where the process starts (threshold).