What this means is that, for particular queries, users will see a list of results, in the right-side area reserved to Knowledge Graph or social results, from their Gmail accounts. These can be anything from received emails to drafts.
The idea is to provide a faster way of discovering information from your Gmail account without having to visit the site or leave the search page.
Google says this is a continuation of the universal search concept it's been gearing towards. The idea is to make any information that may be relevant to you available as fast and easy as possible.