I found this online and it explains pretty well what a compressor does - it`s easier to visualise on a compressor with actual knobs or representations of knobs than with bare numbers but it makes fairly clear what the parameters represent.
"What a compressor does
A compressor/limiter, is essentially an automatic volume control. Imagine an engineer with his hand on a fader and his eyes on an input level meter. As long as the meter stays below a certain point (the threshold), he leaves the fader all the way up and the gain is unchanged. But the instant the sound gets louder, the engineer pulls down the fader by a certain amount. After the sound gets soft again, the engineer will push the fader back up. That's what the compressor is doing, except much faster and more accurately than humanly possible.
Paradoxically, by cutting the peak levels, a compressor allows you to raise the average level of a sound using the Output control and make it sound louder. By using the threshold and ratio controls, you can set a stable sound that will hold its position in the mix whether the singer is whispering or screaming.
What the controls do
Let's go back to the "engineer with his hand on a fader and eyes on the meter" analogy. The front panel controls simply tell the "engineer" what rules he should follow. [THRESHOLD] tells him how high the input meter can rise before he has to start pulling down the fader: if it's turned full clockwise, he won't pull down his fader until the red +6 LED comes on; if it's turned counter-clockwise, he'll have his hand on the fader even before the lowest green -30 LED lights. [RATIO] tells him how far he should "pull the fader down" when the signal is above the threshold level: should he pull it down just a little bit (compression) or pull the fader as far down as necessary to make sure the output level is never higher than the threshold (limiting)? The [HARD/SOFT] switch affects how he reacts as signal approaches the threshold: does he reduce it exactly by the ratio only after it crosses the threshold, or does he gradually ease into the full ratio as it gets close? The red LEDs of the reduction meter tell you how much the "engineer" is pulling down the "fader" at any time. If these LEDs aren't on, his hands are in his pockets.
The [ATTACK] and [RELEASE] controls involve the speed of the engineer's response. Short attack times order the engineer to get his hands on the fader 1/10,000th of a second after he sees a too-loud signal; long attack times tell him to let transients less than 1/5th of a second pass. [RELEASE] tells the engineer how quickly he should push the fader back up again after a loud signal has stopped; when it's turned counter-clockwise, he pushes the fader back up instantly, and when it's full clockwise, he'll take three seconds to push his fader back up to unity gain.
The [OUTPUT] control is simply a gain control located after our "automatic engineer in the box". The [INPUT/OUTPUT] switch allows you to see the levels before the engineer does his job, or after."
Hope that makes things as bit clearer
Will