API Sandbox runs your code in a “sandbox” environment which MS set it up. You can validate your code simply by clicking the “Run” button. Results will be displayed in the “Console” panel underneath the code snippets.
Visit the API Sandbox Here: https://apisandbox.msdn.microsoft.com/
You can read more about it at the following blog post.
If you want to know more about Sample data you can read the following article. This article shows the data that is associated with the sample user in the sample tenant: