RSS is a standardized protocol that allows you to get updates from websites when new content gets published. Using a RSS client, you can follow multiple websites in a single interface.
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.
Really Simple Syndication is a format that allows a program or user to download a bunch of stuff as a simple list of titles, content and tags. RSS readers can also combine different sources into one feed, so you can grab articles from many different RSS capable sites and combine them into one cohesive list.