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 following the instructions provided here - Accessing your PayPal Checkout Client ID and Secret Key
Once you have created a SandBox account, you can test the payment integration of your form by:
1) Access the Payment settings page of your form, and then enter your Test Client ID into the "Test Client ID " field and your Test Secret Key into the "Test Secret Key" field. Please note that you'll also need to enter your Live Client ID in the "Live Client ID" field and your Live Secret Key into the "Live Secret Key" field in order to be able to save the new payment settings.
2) Activate the SandBox Mode by checking the "Testing (Sandbox) mode" checkbox.
3) Review the payment options of your form, assign prices, activate the payment functionality of the form, and click the blue "Save" button.
4) Test your form by using a buyer's SandBox PayPal account.