For all intents and purposes, just think of an RSS feed as the place where all of a podcast's information is kept, as well as an up-to-date list of all the episodes. It's kept somewhere on the Internet, and all we need to know is where.
To find it, go to a podcast's webpage and look around for either the letters RSS or a logo that looks something like this:
Often, your browser will tell you if there is an RSS feed somewhere on the page by showing the above logo in the address bar. Click on that logo, and it will give you a list of feeds on the page.
Once you've found the feed, click on it. You'll be taken to a page that may or may not make sense. What's on the page is not important. What you need is the URL for that page. It might look something like this:
Just copy that URL into the above field and press submit and we'll check to see if you got it right. Good luck!