When you listen to your mixes in the car, do you have to crank the volume dial up way past usual? Are you a little confused by Compression? You might find this video useful. I cover compression basics, including threshold, ratio and make up gain. Setting a compressor boils down to three steps:
Set your threshold so your input is hitting it.
Set your ratio (1:1 is no reduction, 20:1 is LOTS of reduction).
Add some makeup gain to get to desired volume level.