It's basically the websites content in a simplized format called xml. Reader applications such as the ones included in rainmeter use these xml tags to catergorize and then dispalay predetermined content.
So if you wanted a joke feed, you'll need to find a website that provides jokes in an RSS feed and then use that in a reader
OOOOH LOL
why didn't you just say it in Noob Speak for me like Fat0ninja did xD
Edit:
I found a website but I don't know what to do so if you could gimme a step by step what to do thing please xD
[Click me] <-- Website