RSS - sometimes called RSS feed or just feed - is a web format used to find blog posts in an easy and efficient way.
Bloglovin must have access to your blog's RSS feed to be able to find your posts and display them. Since RSS is a very strict format, everything must be formatted correctly. If one bit is off, Bloglovin will not be able to find your latest posts.
You can check if your RSS feed is valid with a feed validator.