What is an RSS feed?

I’m trying to figure out this RSS feed thing. If I want to add RSS feeds to my website, do you have to constantly update the feed? I guess this is what I’m trying to say. When I go to a site and the content is available for RSS feed, I click on it and it gives me a page of code. If I add this code to my site, is it just a static page of news that would have to be manually updated? I’ve noticed that yahoo has a RSS feed that is just a small piece of code that automatically updates for instance, headline news. Can anyone explain all of this to me? Thanks.