This was designed by someone over at the Mozilla foundation. It's used to signify to a user, using Firefox, that the site they are viewing has a feed availalable.
Currently there is no set standard as various sites like CNN, BBC, etc use a "RSS" or "XML" button. Both of these acronyms make no sense to the avg. user, in fact some tests have show that the "RSS" button can be mistaken for "R55" if it's small enough. Talk about confusing the user!
Matt Brett, a designer, has created a site called feed icons
***********************Update on 1/27/06*******************
Let's see, I had to stop typing to take care of my daughter and then work got in the way.
I'm back now to finish the thought (can't wait to have "draft" posts in Journals)
So Microsoft was toying with the idea of their own "feed" icon and finally came to the conclusion that Mozilla had it right.
I think you'll start to see this icon on more and more websites once IE7 comes out.
What do you think of this icon? Does it make sense to you? Do you know what feeds are? Tell me what you think.