A site feed is a machine-readable representation of your blog that can be picked up and displayed on other web sites and information aggregation tools. For information on enabling your feed, see: How do I change my site feed settings?
Special pieces of software called feed readers, news readers, or aggregators can scan your site feed, and automatically let your readers know when your blog has been updated. An example of such software is Google Reader, which you can use with your Google Account. Other options, including both web-based and client-side software, are listed at AtomEnabled.org.