RSS feed is a term you will come across constantly as a blogger. It's an extremely important concept to understand if you wish to expand your blog's audience.
RSS stands for Real Simple Syndication. Don't ask why it's the term used. What's important is what it's used for. Basically, your blog generates a feed when you publish your content. Think of it like a magazine. One blog post is a single issue of a magazine.
The RSS feed is how people can subscribe to your blog. By subscribing to your blog's feed, they are telling the blog to send posts ("issues") to them on a regular basis. The most common methods of receiving a subscription are through email (the blog sends the post as an email) or through an RSS reader (a program that can read RSS feeds).
So why is this useful?
First of all, it's very convenient for readers. Ideally, you want 100% of your readers to read 100% of your blog posts. Unless a blog is part of a person's usual internet browsing routine, it's easy to forget to check back on a regular basis. Having a blog post sent to an email increases the likelihood that it will be read.
The other big reason is that it helps you, the blog author, to expand your audience potential. Many places such as (but certainly not limited to) Facebook allow you to upload your blog's RSS feed. This means that people could view your posts on Facebook. You use the more popular site's broader appeal to help attract attention to your blog.
But before you go upload your blog everywhere do keep something in mind:
If you upload your RSS feed to every site you come across it means that people are reading the post on the third party site and not your blog. The number of people you actually have visiting your blog greatly affects your SEO.
So to sum up: find the careful balance between attracting new readers but at the same time keeping people on your blog.