Finding the RSS URL
Podcasts are syndicated using RSS, similar to the way blogs are syndicated. RSS is really just an XML file specification that allows some of the content to be machine-readable – so, for instance, blog content can be gathered using a news aggregator and can be read offline, instead of having to use a web page.
Podcasts use this same mechanism to tell podcast clients where to find new episodes to download.
The “podcast RSS feed url” is just another name for the RSS feed the podcaster uses to announce shows. It is often denoted by a podcast icon or an orange XML icon, such as the following: