Compression is the reduction of dynamic range in an audio signal. When you see a typical compressor, it has thresholds, ratio, attack, and release.
The threshold indicates where the compressor begins compressing the signal. This means setting the threshold to 20dB will only compress the signal once it reaches 20dBs.
Ratio is the amount of power the compressor removes from a signal. This means a 20:1 amount of ratio would reduce way more from the signal than a 10:1 amount of ratio would. So for example a 10:1 compression ratio compresses ever 10dB over the threshold. The difference would be more audible than visual. However you can control the Knee of a ratio.
It is the amount of time the signal takes to compress after reaching the signal threshold and circulate through each compression. Therefore, the longer the attack time the longer the original attack before the compression activates.
The release correlates with the attack. It tells the compressor when to release the signal from compression. This means that once the signal goes down the threshold set, the release will activate.