Other software can download an RSS feed (usually are regular intervals, perhaps even every few minutes), examine it for new content, then notify a user or perform some other programmatic action on it. Most blogs have RSS feeds.
RSS may stand for any of the following:
Rich Site Summary
RDF Site Summary
Really Simple Syndication
ATOM is a different, but highly similar, format. Many sites will offer both RSS and ATOM feeds in parallel, and many tools consume both. ATOM feeds are often referred to as “RSS”.