The sandbox is a part of your wiki that you can freely modify. It's meant to let you practice editing. You will discover how page editing works and create new pages. Simply click on Edit to get started!
Don't worry about overwriting or losing stuff when editing the page, you can always roll back to the first version of the page from the "History" tab at the bottom of the page.
If you want to give a look to the underlying XWiki Syntax, you can click on "Wiki code" in the "Show" menu or click on the "Source" tab when editing the page.Here are a number of test pages you can play with: