Website feeds are often used to publish lists of site updates, but they can also be used to provide other structured information - for example, search results or product lists - that readers can find useful to track. Readers can subscribe to these feeds and stay up to date with changes on your site. Feeds let readers keep track of a large number of sites or blogs without having to check each site manually. If you create your site using a tool such as Blogger or Typepad, you'll probably have a feed automatically created and published for you.
Subscriber stats display the number of Google users who have subscribed to your feeds using any Google product (such as Reader, iGoogle, or Orkut). Because users can subscribe to feeds using many different aggregators or RSS readers, the actual number of subscribers to your site may be higher. Also, these stats list only those feeds with actual subscribers. If your site publishes a feed that does not yet have any subscribers, it will not be listed.
You may find that the subscriber numbers in Webmaster Tools may vary from those in your FeedBurner account. This is because Webmaster Tools subscriber stats reflect known subscribers to feeds hosted on your verified domain (www.example.com). FeedBurner sees all requests for your feeds, whether these requests are made to www.example.com/feed or feeds.feedburner.com/yourfeed. If a Google user is subscribed to the FeedBurner version of your feed (for example, http://feeds.feedburner.com/yourfeed or feeds.example.com/yourfeed), this information will not be reflected in Webmaster Tools subscriber stats, but it will be included in your FeedBurner stats.