What is high idle?

At the risk of exposing my "Newbosity", I have to ask what the "high idle" is? I've seen people say "you should get the high idle kit" or "should I get the high idle kit?" What the heck is the high idle, is it a problem that needs fixed, is it a performance upgrade. Obviously it has something to do with the CTD's idleing higher but that's all the further I got on my own. I did many searches, and the only results I get are where people mentioned the high idle, but I couldn't find a good explaination of it. Could someone point me in the right direction? Moderator's, please put this in the appropriate place. If I knew what high idle was, I'd put it in the right place, but then again, there'd be no need for this thread at all. Thanks in advance.