The basic principle is that if a user decides that a web site such as the BBC News web site or a blog, has regularly updated content worth following, they can ‘subscribe’ to it. This involves finding the RSS feed (a kind of web address) for that site usually represented by an orange icon.
The users then copies the feed into something called an RSS reader (aka RSS aggregator) which displays the titles of new articles or posts as well, generally, as the first few lines. The user can then skim read the list that it produces. Clicking on any item in the list will take them to the full article. RSS readers include Google Reader but I like personal start pages like iGoogle, Netvibes and Pageflakes which also include this functionality.
Here's another great YouTube video definition:
RSS in Plain English