What are RSS Enclosures in Outlook 2007? What does the "Automatically download Enclosures for this feed" checkbox do in the RSS Feed Options dialog?
an RSS enclosure in a feed's post is equivalent to an email attachment.
Podcasts for example will have the audio file as an enclosure in the posts.
The option does exactly what it says on the tin and will automatically download any attachments in new posts when your feeds update.