When integrating the SDK, a value for both the shopperResultUrl and the notificationUrl are required.
The shopperresulturl is the location where the customer is redirected to after payment, this is likely a location in your app. Usually on this page you run the Get Payment Status call to decide if you use a successful payment page or unsuccesful payment page.
The shopperResultUrl is configured in your app, see: https://peachpayments.docs.oppwa.com/tutorials/mobile-sdk/first-integration
The notificationUrl is an endpoint on your server that listens for payment status changes. This mechanism works as a fail safe, so that, if the customer is not redirected back to your app, you still receive the actual payment response on your server.
This notification has the same layout as the payload that you'll receive with the normal GET request.
The notificationUrl is configured in the parameters when you setup the checkoutId in step 1. See also: https://peachpayments.docs.oppwa.com/tutorials/mobile-sdk/integration/server