The Debug -> Output tab is displayed only when output debugging is enabled. This can be achieved either by configuring certain settings in Site Manager -> Settings -> System -> Debug, or by adding specific keys to the appSettings section of your project's web.config file. Both options are described in this context help topic.
On this tab, you can see the exact output code of recently displayed pages. This is particularly useful in case of AJAX request, whose code can't be viewed as part of the page source directly in the browser.
Enabling the Show complete context check-box displays complete context (not only the topmost item) in the Context column. The log can be cleared using the Clear output log button.
For more information about debugging in Kentico CMS, please refer to Developer's Guide -> Development -> Debugging and system information.