In your switches you have an option to activate the search log. Once activated, all searches performed by your visitors will be recorded. The most frequent use of search log data is to display a list of the most frequently searched terms. The 'search log' option on your admin panel menu (it's near the bottom once you switch search logging on) does this, as well as allowing you to find how many searches there've been for any particular term.

If you want to display the data in some other way or somewhere else you'll need to create a toplist -- go to your toplist generator and the search log option will appear when the switch is enabled.