An RSS Feed is a way of syndicating, that is distributing content, or updates to websites (such as new blog posts) easily across the internet. A Feed is formatted in a certain manner, according to Internet standards, which is supposed to not change between different feeds. A Feed can be read by a feed reader, to allow users to easily view updates to a website for example, all directly from the feed. You can see an example of a feed by clicking on the 'Subscribe to' button on this site.
There are many different types of feeds such as Atom, XML, and RSS, each with their own unique format and standards.