What is it?

Offline access in Outlook Web App for Exchange 2013 lets users use Outlook Web App even when not connected to a network.

Offline access is newly available in Outlook Web App on the following web browsers.:

For more information about the offline user experience, see Using Outlook Web App offline.

--> Please see the rest on the blog : http://blogs.technet.com/b/exchange/archive/2012/11/06/offline-access-in-outlook-web-app-2013.aspx