What is a notification payload?
Payload = sending extra details with messages. With every message you only see title/message. That's it. But what If I want to send to the app extra information so that the app can re-use it to do something or customize how the message looks like. Like for instance, I want the user to open a certain screen in the app, or visit an external URL, or display a big picture, or ....etc.
Aha, how do I send this payload?
Just head over to your app and click push. Then scroll down for the Payload section and start adding keys and values.