I use NetNewsWire on my iPad and Feeder on my Android phone to read RSS feeds from multiple sites (including Lemmy and Kbin). I also use it to monitor niche sub-reddits so I can give Reddit as little traffic as possible.
I guess it was inevitable that Reddit would know many are doing this and begin to kill RSS feeds. I’ve noticed the past few days that my feeds were not being updated very often, if at all, even though the feed appears to still be there.
This started a few weeks ago, apparently:
https://www.reddit.com/r/bugs/comments/14z44nw/rss_feeds_are_broken_for_most_subreddits/
In a very simple manner, is a file that contains all the content that a website (in this case a subreddit, but it can be a blog for example) publishes. For each publication, the RSS file contains an entry and each entry contains information like the author of the publication, date, content, summary, media links and so on.
You can use an rss reader to aggregate different RSS feeds from different sources and read them from a single app.