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Attack - The attack knob tells the compressor how fast (or slow) to compress the signal once it hits the threshold.  This is useful if you want to maintain some of the "punch" and dynamics of a signal, but still want to control its volume.  Let's say that you have a kick drum that you want to preserve its initial transient (the first part of the hit), you can dial your attack to start a little bit after the start of the drum hit happens, and then compress from there.  This will in turn let the initial hit of the drum pass through uncompressed, and then start compressing at the designated time you have set the attack to.  So, if you set the attack to 20 ms, the compressor will wait 20 ms AFTER the signal reaches the threshold, and then once that 20 ms has passed, it will enact the compression.    


​Release - As you can imagine, this control tells your compressor how fast (or slow) to end the compression after it no longer reaches the threshold.  So, if you set your release to 150 ms, that means that the compression will end or "release" 150 ms after the signal goes below the threshold.

​Ratio - Ratio is the amount you will allow the audio signal to go above the chosen threshold.  

So, if your ratio control is set to 2:1, you flip 2:1 upside down, and turn it into a fraction = 1/2. 

Ratio and Threshold Work Together.

So, if the threshold is set to -6db, and your ratio is set to 2:1 - when the audio signal gets up to -6db, the compressor will only allow 1/2 of the original signal above the threshold.  If the ratio is set to 4:1, it will only allow 1/4 of the signal above the threshold.  If the ratio is set to 6:1, then only 1/6 of the signal will be allowed over the threshold. If the compressor is set to infinity, then your compressor has been turned into a limiter, and it won't let any sound above the threshold. 

The higher the Ratio, the more aggressive the compression.
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Check out my handy-dandy Compression Ratio Chart for reference: