RSS is a live feed source from the internet. It can basically be whatever is out there. News, forums, entertainment...all kinds of stuff.
vBulletin 3.6.x and up has built in RSS functionality and you should have a group in your admin CP that houses it. You basically create a forum, find a feed on the net and put in the URL under the settings.
So no modification needed unless you wanted to do a block for CMPS or something.
You can see an example of mine
here
That is the center column for the CMPS mod (wonderful) so that I could show them on a separate site page from the forum. How you want to do it is totally up to you.