This is an interesting way to browse the history of statements by U.S. Presidents through history. A "tag cloud" is a technique for visually presenting the frequency of keyword use. With a given set of text, looking at the frequency of use for the key words, if you enlarge the most commonly used ones then it's visually obvious the thrust of the text.
For example the State of the Union Address
George W. Bush, 31 January 2006 the most common word was Terrorists. In that speech GW Bush wanted to again drill home to the population the fear mongering stance he's taken since September 11, 2001 to convince us we need to support his cause because we're in fear of the Terrorists. And the relative size of the words makes this obvious with Terrorists being most commonly used, and Iraq next so.