What is sandbox mode?

Sandbox mode is a testing mode that mimics the live production environment for PayPal transactions. Its sole purpose is to conduct test transactions using assigned email addresses that you would get when you create a Sandbox account. 

You can create a SandBox account here:
https://www.paypal.com/webapps/merchantboarding/webflow/unifiedflow?execution=e1s2 

You can read more about it on this guide:
https://developer.paypal.com/webapps/developer/docs/classic/lifecycle/ug_sandbox/ 

I hope this helps. Do not hesitate if further assistance is needed.

Thank you