what does the symbol “?” mean and how can i edit that page?

? means the start of the variables that are accessable via PHP on the page called. The page https://www.testsite.com/ will be called, and booking variable will be accessible globally throughout the PHP script. In this case, the homepage script will be run. The $_GET['booking'] index will be set (to NULL), and will (usually) be accessible also at $_REQUEST['booking'].

Now, this URI could very well be a dead link if you found it published somewhere in your code. In the case that it does represent a post or page, you can edit it in the back-end admin panel by clicking on either “Posts” or “Pages” in the left hand admin column. After doing so, you will get a list of the posts/pages on which you can click on its appearance in the table and edit its permalink.