The sandbox is a pre-packaged Nette Framework project, basic configured structure for your application.
This is a simple pre-packaged and pre-configured application using the Nette that you can use as the starting point for your new applications.
Nette is a popular tool for PHP web development. It is designed to be the most usable and friendliest as possible. It focuses on security and performance and is definitely one of the safest PHP frameworks.