RSS (an acronym for 'Really Simple Syndication') is an XML-based file format for providing well-defined syndicated content to "feed readers." Stack Exchange sites provide RSS feeds of content available on many of its pages. For example:

• /feeds Recent Questions
• /feeds/user/ID User Activity
• /feeds/featured Featured Questions
• /feeds/tag/TAG Tagged Questions
• /feeds/question/ID A Particular Question

See also What other hidden or inobvious RSS feeds are available on Stack Exchange and its sites? for an elaboration on the different kinds of RSS URLs.