Click the Statistics tab under site search to view your visitors' search terms. Here, you can view all the terms that visitors entered into the search bar. Use the drop-down menus to show only certain date ranges and the number of terms you want to view. Click the term to see the results that show for it (this is what your site visitors see).
The boxes divide the search terms into those that returned actual results, versus those that didn't return any results. Visitors see no results (or "zero matches") when their entered search term doesn't appear on any of the pages that you selected to index. It can be useful to know which terms don't return results so that you can create content for those terms if needed.