RSS is just an XML file, one of the tags is an enclosure tag, that tag specifies a link to an MP3 file of a new audio program that the author has posted online.
When the iPodder aggregator is run it checks for new MP3 files and downloads them into a playlist (Windows Media, iTunes, ect) that you can later sync up to a portable player.
Why is this needed? hell that should be simple for anyone using an aggregator at all. Its just easier to have this stuff come to you then for you to go out and be on the look out for new 'IT Conversations' or 'The Daily Source Code' episodes.
Yes the concept is simple, and very basic but it didn't exist before, so this fills a hole that a lot of listeners to these types of shows really needed.
After reading some of the comments about iPodder, I was more than a little confused on how so many people could wrap their heads around what iPodder does, I hope this helps (sorry that is late in coming)