RSS stands for "Really Simple Syndication". It is a way to easily distribute a list of headlines, update notices, and sometimes content to a wide number of people.
Most people are interested in many websites whose content changes on an unpredictable schedule. Repeatedly checking each website to see if there is any new content can be very tedious. RSS is a better way to be notified of new and changed content. Notifications of changes to multiple websites are handled easily, and the results are presented to you well organized and distinct from email.
RSS works by having the website author maintain a list of notifications on their website in a standard way. This list of notifications is called an "RSS Feed". People who are interested in finding out the latest headlines or changes can check this list. Special computer programs called "RSS aggregators" have been developed that automatically access the RSS feeds of websites you care about on your behalf and organize the results for you.
RSS provides very basic information to do its notification. It is made up of a list of items presented in order from newest to oldest. Each item usually consists of a simple title describing the item along with a more complete description and a link to a web page with the actual information being described. Sometimes this description is the full information you want to read (such as the content of a weblog post) and sometimes it is just a summary.
Think of an RSS aggregator as just a web browser for RSS content. RSS aggregators automatically check a series of RSS feeds for new items on an ongoing basis, making it is possible to keep track of changes to multiple websites without needing to tediously read and re-read each of the websites yourself. They detect the additions and present them all together to you in a compact and useful manner. If the title and description of an item are of interest, the link can be used to quickly bring the related web page up for reading.