In a nutshell, a compressor is similar to riding a fader on a mixer, only automatically. The Threshhold controls when the compressor kicks in, the Attack controls how fast the compressor kicks in, the Ratio controls the amount of compression, the Release controls how long (time) the signal is compressed, and the Gain is like the overall output level knob.
Here’s a quick "nutshell" link -
http://www.ethanwiner.com/compressors.html