The site's RSS feeds refer to the automated systems that post updating pages into chat. This tag should be used on questions that directly relate to those chat feeds.
RSS feeds on the network are an automated system to keep track of updating pages such as the new questions feed. These feeds are often used in chatrooms. As a result, questions that discuss RSS feeds should often be tagged with chat as well.
A typical question about RSS feeds here could be asking for a feed in The Nineteenth Byte, asking for a feed to be removed or suggesting features and bugs specific to the feeds.