There are usually only a few knobs on the front panel of a compressor which is nice considering most studio gear comes with huge instruction manuals and fifty knobs and parameters to keep up with. First of all most compressors, like other equipment, will have input and maybe even output levels. These are just to make sure that you have a strong signal going in and out of the unit. Output level should not be confused with makeup gain, which we will get to in a bit. Next are the meters.
Meters are a powerful, visual tool to measure input signals as well as gain change at the output.
Some units may have separate meters for this and others will have a selector switch to toggle between the two. Threshold level tells our imaginary sound guy at which point you want to start backing off the level. This parameter will greatly effect all the others because it basically tells the unit when to go to work. All though it may not always jive, the increments are designed to follow the fader position on the mixer. Look at your input meter on the compressor and play a track through it.