UPDATE: December 2018
After one year of testing, we have decided to shut down the beta of Sandboxes. Using the feedback that we gathered, we are currently working on a new and improved version of the test environment feature. Make sure to follow us on Twitter for news and updates on the release date.
Sandboxes act as stage servers, allowing you to run your application from a chosen branch with just one click.
Every time you make a push to this branch, the application is rebuilt and run afresh. It's a great solution for stage branches because the sandbox will always serve the newest code version.