1- Transaction processed callback: 

It's an URL for an endpoint in your web app, on which you'd receive callbacks as a notification after performing any payment process from your customer side, you would receive a JSON object in POST request which contains a value by which you can know about your payments such as the status of the transaction (success/declined), the order ID related to this transition, the transaction ID and much other information related to your transaction.

2- Transaction Response callback:

After performing any payment you should redirect your customer again to your platform, on the transaction response callback you would receive query parameters which you can parse it and upon its value, you'd show your customer a proper message. However, if you're using our Android or IOS SDK, this part could be handled from the SDK itself.

- PS: For more info and samples, kindly check this link here.