Ratio
The ratio sets the amount of compression that is being applied when the threshold level is being reached. The settings are set in ratio's like 8:1.
When there is 8db of signal going over the threshold level this will be reduced to 1db when using this setting. So 8db over the threshold level becomes 1db over the threshold level.
The ratio is very important because it sets the power of the compressor.

Threshold
This is the level at which compression will set in. If a signal doesn't pass this level the compressor won't touch it. But if it does exceed the threshold level compression with the power of the ratio will be applied.

Attack
The attack sets the time it takes for the compressor to set in after the threshold level has been reached. If this is set to 0 the compressor will directly work when any signal is passing the threshold level. But if set higher it will gradually set in after the threshold level has been reached. This is useful if you want to maintain the initial attack of the sound you are compressing.

Release
The release sets the time it takes for the signal to get back to it's original level after it has dropped below the threshold level.
If you want to have your compressor to pump up your sound you need to set a fast release. Now pull down your threshold until you reach the desired effect without clipping. If you haven't reached the amount of 'pump' you like but the signal is already clipping you should set your ratio higher. In this case you could also set your attack faster to get to the loud unprocessed attack earlier.

;)