A Really Simple Syndication (RSS) feed is a special kind of web page containing a summary of content available from a web site. RSS feeds can be read automatically by special software, known as an "RSS reader" or "aggregator" which automatically reads the RSS feed for new content and can automatically download any updated content. Some web browsers and email clients have built-in RSS readers.
"Subscribing" to an RSS feed means to program an RSS reader to begin reading an RSS feed. There is no cost involved.