Since millions of people across the globe already use mobile phones like there's no tomorrow, Google launched Mobile Web Search with support for many languages.
Try it the next time you visit Google on your mobile phone - you'll see a new option to search the Mobile Web. How different is it than standard web search? There are sites out there that have already been designed for your mobile phone, which makes them more navigable on the small screen.
So Google created an index specifically for these sites. And so your phone can now be that much more useful.