The UI search box allows you to search through the interface to track down some of the more hard to find options or features in Document! X / HelpStudio. Performing a search is as simple as typing in what you are looking for in the search box; you will then be shown a list of matching results. Clicking on any result will open that part of the interface and highlight the matching interface element.
More information on the UI Search can be found in the Innovasys blog here: http://innovasys.wordpress.com/2012/11/12/help-and-support-improvements-in-2012-1-98/