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:
/feedsRecent Questions/feeds/user/IDUser Activity/feeds/featuredFeatured Questions/feeds/tag/TAGTagged Questions/feeds/question/IDA 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.