The PayPal Sandbox Mode is a self-contained, virtual testing environment that mimics the live PayPal production environment. It provides a testing environment where you can test the payment integration of your form without touching any live PayPal accounts.
Please note that you must use a SandBox PayPal account in order to test your form under SandBox mode.
You can create a SandBox test account by:
1) Use your existing developer PayPal account to log into the Developer website at developer.paypal.com, or create a new developer PayPal account by clicking the Sign Up button on developer.paypal.com
2) Create a set of virtual Sandbox test accounts - https://developer.paypal.com/docs/classic/lifecycle/sb_create-accounts/
3) Add the email address associated with your PayPal SandBox account in the Email Address field on the Merchant Setup section of the Payment Settings page of your form.
4) Activate the SandBox Mode by checking the "SandBox Mode" checkbox on the Merchant Setup section of the Payment Settings page of your form.
5) Test your form by using a buyer's SandBox PayPal account.