This tab is displayed only when one of the bulk keys described here is used or when the following key is added to the AppSettings section of your web.config file:
<add key="CMSDebugSecurity" value="true"/>
On the Debug -> Security tab, you can see which security checks were recently performed on the site. This is particularly useful if you want to quickly find out why some user is not able to access some section of the UI or gets the Access denied page displayed.
The log can be cleared using the Clear security log button.