An endpoint is a connection between your App and a Tines story. It's how data flows in and out of your App.
There are two directions to think about:
Sending data to a story. When someone uses your App, say, they submit a form, that information needs to go somewhere to be processed. An endpoint passes that data into a story, which can then take action on it.
Receiving data from a story. Sometimes your App needs to display live information, like a list of open requests or the status of an order. An endpoint lets your App pull that data back from a story and show it to the user.