Now on Tab is an additional feature of the Google Now family. With a press of a button (or a voice commend) Now on Tap does a contextual search within the app you are currently in and presents you with suggestion cards. Cards could be informational, such as a link to a movie trailer you were discussing, or action items as ordering a cab or creating a reminder.
To turn on Now on Tap hold the Home button, and follow onscreen instructions. To use it, simply hold the home button on the screen you want to get additional info for.