Can Someone explain the Sidechain feature in the Pro Channel Channel Compressor?

It makes the key input to the compressor available to other sources rather than the incoming signal on the current track. So you can use say the kick drum output to key the compressor on the bass track this effectively can compress the bass in time with the kick drum. Can be used for ducking, or just building a sympathetic response to another rhythm source in your track.

No, the kicks compressor doesn't control the bass. The bass compressor has the side chain turned on which makes it accessible to other inputs which can control it. You have to start your process with the question, "What needs to be controlled?". Then turn on the compressor sidechain for that target. It now exposes the compressor input to other tracks for input.  

Truthfully...I do not understand the concept of sidechaining. Can someone point me to how, why and where and when to use it? ...or maybe point me to a good source of info on it? Thanks!

Basically, sidechaining allows you to trigger an effect on one track by using the sound from another track.
 
In other words, you can make a synth pad pulse in time by sidechaining it to your kick track. Every time the kick drum hits, the synth pad will momentarily drop in volume and then come back up. You hear this effect all the time on dance music, and more-and-more on pop hits.
 
Here's a great Sound on Sound article that explains the details. http://www.soundonsound.com/sos/nov06/articles/sidechain.htm

Good sources above but just a couple of comments.

1. Can be used for ducking a track or group. Radio does this when you hear the voice come forward and the music behind go down, but just when the voice is talking.
2. Can be used when two instruments compete for frequency range and you want one turned down slightly when the other is active. i.e. the kick for some types of music needs to be in your face. Duck the bass guitar which is in the same frequency range when the kick strikes. This gives you consistent punch.
3. Frequency specific ducking. Put an EQ in front of the compressor so that only a specific range is available when the compression starts. Let's say you want an acoustic guitar to duck slightly in the vocal range keyed by the voice or even the frequency range of another guitar.  
4. Frequency specific ducking for reverb to control the reverb when a voice comes in loud.

Lot's of uses. But the kick/bass and voice vs. music are the generally the most often used.

Middleman



3. Frequency specific ducking. Put an EQ in front of the compressor so that only a specific range is available when the compression starts. Let's say you want an acoustic guitar to duck slightly in the vocal range keyed by the voice or even the frequency range of another guitar.  
4. Frequency specific ducking for reverb to control the reverb when a voice comes in loud.

Frequency specific compression is also what de-essing is. Sidechain an EQ to a compressor and decrease the sibilant sounds in a vocal track.

oops

Or listen to this tune... don't forget to listen...

http://www.youtube.com/watch?v=L_fCqg92qks&ob=av3e

Smedberg


Or listen to this tune... don't forget to listen...

http://www.youtube.com/watch?v=L_fCqg92qks&ob=av3e

That IMO is overdoing it a bit...not sure I like it when everything in the mix is ducked like that...kind of makes my eyes water.

Another use for side chaining is to duck an echo with a lot of feedback on a guitar solo.

Another classic example would be Daft Punk - One More Time.