On most compressors, the graph is represented as such:
x-axis = input volume
y-axis = output volume
So let's say you do 20:1 compression at a threshold of -10db. On the graph you'll see a nice linear line right up until -10db, at which time the line will shoot off horizontally very quickly. This is because for everything under -10db, the compressor will not change the output volume (disregarding release), but after -10db, the compressor will begin to compress at a ratio of 20:1. Hence everything over -10db is going to come out real close to -10db (disregarding attack) in this example (because the compressor will output +1db of gain for every +20db over the -10db threshold).
That's the best I can do, if you want some real compression discussion, you might ask over in the mixing forum.
Slackmaster 2000