RSS stands for Really Simple Syndication.
Back when blogs were a bigger thing, they would be setup with RSS to “push out” notifications when new posts were published. (Technically your RSS client pulls the RSS feeds but the end result is the same - the feed is just a list of posts basically).
You open up your RSS client or site and there will be a list of sites you’re “following” and any new posts they’ve made.
Plenty of sites still support RSS. A lot of readers can pull the RSS feed automatically if you just give them the site URL/web address.
My personal choice is NewsBlur which is at https://NewsBlur.com. You can get a free account there to try it out.
My feeling is they looked at their user numbers, and the specifically those using the API and/or 3rd party apps, and did the calculation to decide that they wouldn’t lose enough people to cause a mass migration.
I think whether that calculation was correct or not still remains to be seen.