The Summary tag was exactly that, a summary description of your podcast. This is the elevator speech iTunes Music Store visitors read before checking out your podcast or ignoring it for something more compelling, so make it count. HIstorically this was the only iTunes custom RSS tag without a 255 character limit. Think about what you would say to convince someone in 15 seconds or less why your podcast is awesome and use that text for the summary. Later, as Apple moved to a standalone Podcasts app, Summary was replaced by the <description> element in an RSS 2.0 channel definition, making summary redundant.
Your summary might look something like this: <itunes:summary> In every episode, the MediaBlab podcast covers consumer electronics tips and technology hacks. Learn about VoIP, HDTV, Digital Video and more.</itunes:summary>
