RDX stood for Raid Data Exchange. Originally it was a CTRaid replacement + CTBossmods all rolled into one. It had the ability to modify bossmods on the fly and the ability to send them to other raiders in game without the need to have them go download something new. So you are pushing content, a boss has a ZOMGPOUNDYOURASS skill and the programmer would write it up to warn everyone and push it to the raid.

Why is it not documented well? Because the guy who wrote it was the ONLY guy/guild using it for a LONG time. Eventually this leaked, a few chosen friends got into using it, and of course eventually it bled out to the mass public. RDX5 Conquest was leaked, this wasn't the core version but the version Conquest was using, I picked up that leaked copy and maintained my own guilds version for about a year before RDX6 went into public Beta (payed mod) and I signed up for that almost immediately.

The core RDX is very much like InnerSpace - it is a platform. InnerSpace by it's self doesn't do anything, it just sits there and you look at it, and it looks at you. Once you get to know each other you can do amazing things with it. RDX is just that, it's a platform that allows you to build bossmods, unit frames, raid frames, logistical data, etc. And it allows you to sync all this across the raid quite easily if everyone runs it. I started using it for raiding, I still use it because I can update my unitframes mod and push it out to my other 4 shamans and they ALL have the same setup, guaranteed.

A great place to start for an RDX Noob would be here

http://www.wowinterface.com/portal.php?id=478 - the base page. Good info, links to other pages.
http://www.wowinterface.com/downloads/cat115.html - downloads page - packages listed here.

From the package page you check OOBE packages, this is "out of box experience" which is basically a bunch of work done for you already. I use the OpenRDX: Brethren RaidFrames package because it's solid, well documented, and flexible.