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.
Yep Cloudflare protects against classic DDoS (like many clients doing a lot of small requests). Here attacks are performed presumibly by users that know very well how the Lemmy's backend works and where bottlenecks are, so that with a small number of well made requests they are able to mess up the backend and Cloudflare doesn't notice it