Postback
A postback is the event Meta's messaging API sends to an app when someone taps a button in a DM — for example, a button on a generic template card. The event carries a payload the app chose when it created the button, telling the automation exactly which option the person picked.
Why it matters for creators
Postbacks are what let a DM conversation branch. When your message offers "Send the checklist" and "See the full course" as buttons, the postback tells the automation which one was tapped, and the flow continues down the right path — no guessing at what someone meant by a typed reply.
They're also clean data. Every tap is a definite, recorded choice, so you can see where people move through a funnel and where they stop — which is hard to read from free-text conversation alone.