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.
It's also important that the page URL you're adding to Bloglovin', features the RSS feed within it's code. If you're not sure how to check this, you'll want to work with your blog provider, or whoever helped you with the technical aspects of your blog.