See important update.
From Search Engine Roundtable, May 26:
Google Adds Personal Tab To Search Filters
Google now lets searchers filter their search results by "personal" results. By that, it mostly means emails, flights, anything related to that Gmail account they are logged into. You should be able to see it if you are logged into a normal Gmail account and do a normal Google search. If you are logged out or logged into a Google Apps (GSuite) account, you won't see it.
Here is a screen shot of the option...MORE
Update: This option has been removed as of Tuesday, May 30th.