RSS, which stood for several different things, depending on who you asked (Real Simple Syndication was probably the most popular), has withered away. It was designed in the early days of the Web to make it easy to process and read news articles and blog posts from many different sites. Once you subscribed to a Web site using your RSS reader app, you could easily browse and read all the content from that site.
This article is a very detailed history of the technology. For some of us early users of the Internet, the article is a trip down memory lane.
There are many reasons why RSS has faded away, but Facebook could be primary factor. Facebook provides the same "endless news feed" that RSS does/did, and bundles all sorts of other information for you.
It is unfortunate that RSS never really took off. It gives the user much more control over what you read, unlike Facebook, which shoves all sorts of unwanted stuff into your feed. The Internet's domination by monopoly giants like Facebook, Google, and Amazon has become a corporate playground where users are the product. It's not a good thing.