Google is always good at providing better search result for our queries. And it always gets better and better. This is a another application in Gmail that allows you to search through all your application ( eg: Google Docs ) from one place.
"Once you enable it from the Gmail Labs tab under Settings, the “Search Mail” button in Gmail will say “Search Mail and Docs” instead, and your search results will include matching documents and sites in addition to email messages." Says Gmail Blog New in Labs: Find docs and sites quickly with Apps Search - Official Gmail Blog