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Threshold is the level at which the compressor becomes active. For small amounts of compression, this is set to touch the peaks of the signal, for greater compression it is brought further down into the audio.
Ratio determines the amount of gain reduction. The higher the ratio, the greater the audio is compressed once it crosses the threshold value. For example, a 2:1 ratio will give a gentle reduction of the signal, 10:1 is aggressive and ratios above that come close to the effect of a Limiter.
Attack sets how quickly the compressor applies the reduction ratio once it crosses the threshold. It can been thought of as a sharpness knob.
The setting of this value depends on what you are compressing. Percussive sounds have very sharp initial transients as they are hit. If you set a fast attack time, it will almost instantly compress this characteristic and deaden its impact. Using a longer attack time will allow the punchy section of the signal through first, then apply the compression. Sounds without an initial hit such as vocals or bass can make use of the fastest attack times, ensuring a smooth, even signal.
Release controls how quickly the compression stops acting on the signal. It can be thought of as a smoothness knob.
Using percussion as an example again, this comprises of multiple hits and impacts. Therefore you want the compressor to do its work then release and reset before the next hit arrives. This therefore needs a fast release time, corresponding to the time between transients at most.
For non percussive parts you can use longer release times to control the signal slower and more smoothly. Again vocals and bass instruments are desired to be smooth by default so benefit from longer release settings. A good starting point is slow attack and fast release for percussion then fast attack and slow release for bass.