Jetpack’s Mobile Theme displays your content in a clean, uncluttered interface, making it easy for mobile visitors to scan your site. Furthermore, we took special care to make the mobile theme as lightweight as possible to ensure faster loading times.
from: Jetpack for WordPress
Activate Jetpack plugin, if not already activated Dashboard->Plugins
Activate the ‘Mobile Theme’ Module- Dashboard->Jetpack-> Settings
There is a configuration screen than can be accessed from the Jet Pack settings by clicking the configure link beside Mobile Theme.