What is PayPal Sandbox?
The PayPal Sandbox is a testing environment within which you can test all the PayPal features. The PayPal Sandbox is an almost identical copy of the live PayPal website.
You should test the payment configuration with the Sandbox environment first to make sure that it functions as you want.
How to create PayPal sandboxes?
Complete details on using the PayPal Sandbox, and create Sandox accounts are detailed here: https://developer.paypal.com/webapps/developer/docs/classic/lifecycle/ug_sandbox/
To plan correct tests, before creating account, follow those Tips and Tricks
Read Account naming conventions
Sign up an account https://developer.paypal.com/
Create Sandboxes for the merchant (Business account) for the country you want to test
and for the client (Personal accounts) https://developer.paypal.com/webapps/developer/applications/accounts
Create one buyer and one merchant PayPal Sandbox account