Google has added the Mobile Usability feature to its Webmaster tools with an objective of keeping users informed on issues that mobile users might see across their websites.
What does this new feature do?

The Mobile Usability feature shows issues it has identified across a user’s website, with all the infographics such that a user can see the progress that he has made. Users can easily read and use a mobile-friendly smartphone by simply scrolling down and up. Users who are not able to see some of the content while they are reading should employ the Mobile Usability which shows the following issues: Flash content, missing viewport (a critical meta tag for mobile pages), tiny fronts, fixed-width viewports, content not sized to viewport, and clickable links/buttons which are too close to each other.

Users should also visit the Webmaster Tools and consider how they would like to resolve such issues; more information regarding how to make a great mobile-friendly website can be found on Google’s Web