compression (sometimes referred to as audio level compression) is the process of reducing the dynamic range of given programme material.
compressors accomplish this task using several common control variables/settings:
all signals coming into the compressor that are above the threshold will be compressed.
the ratio setting specifies how much gain above the threshold you would need to raise the signal by 1dB.
a setting of 2:1, for example, means you would need to input audio 2dB above the threshold to further get an increase in gain of 1dB.
for a setting of 4:1, you would need to exceed the threshold by 4dB to get this same 1dB increase in gain.
settings of 10:1 and greater are generally considered 'limiting'.
compression and limiting occur on a continuum and there is no precise defined point where compression becomes limiting.
if it is so difficult to get the output from the compressor to exceed the threshold that it rarely happens, that is considered to be a limiting setting (and that's why it's called 'limiting' because it literally 'limits' the signal to stay very close to the threshold).
for a 10:1 setting, again, you would need a peak above the threshold of 10dB for an output of just 1dB.
the attack setting determines how quickly (or slowly) the compressor reacts when the threshold is reached.
the release setting determines how quickly (or slowly) the compressor lets the programme material return to the non-compressed state after the threshold has been exceeded.
the knee setting is a measure of how radically the compressor reacts as the material is approaching the threshold.
a 'soft' knee setting is a gradual reaction (like a curve, graphically) and a 'hard' knee setting is more radical (like an angle, graphically).
makeup gain or output
this control is a way to make up for lowered programme level post compression.
and these are some of the basic controls on most compressors.