The Mobile device redirection web part redirects mobile device visitors accessing the page to a specified URL. Two different redirection URLs can be specified based on if the device is recognized as a large or a small device. The typical use is when creating a dedicated website section for mobile devices. You can place the web part on the website's main Home page and redirect mobile users to the Home page of the dedicated mobile section.
More information about can be found in Developing sites for mobile devices.