After payment, the buyer needs to redirect to success page of the merchant website. In that page, payment data with status should be shown to the buyers. To redirecting buyer with payment data automatically, some configuration is needed on the PayPal business account.
PayPal IPN is a notification service that used to notifies merchants about the PayPal transaction. Many times you encounter the problems that PayPal IPN (Notify URL) is not working for sandbox account. To solving this problem you needs to edit Instant Payment Notification setting in your sandbox account.
At the time of integrating PayPal payment gateway in the website, you needs test the transaction on sandbox environment. In that case, some configuration is needed for redirecting buyer back to the website and receive the payment data.