The Discourse API is what is used to pass data between the backend Ruby on Rails Discourse application and the front end Discourse javascript. The API is available on all Discourse sites.
A simple example of the API on your site would be to go to this URL: https://ders.co/latest.json. It shows you the JSON data that is used for putting together your site’s latest topics list. You could use that data on another site by making a GET request to https://ders.co/latest.json. You could use the data returned from that request to recreate the latest topics list on another site.
For some details about the Discourse API, have a look at these topics:
- Discourse API Documentation
- How to reverse engineer the Discourse API (this topic gives a good approach to figure out what API call to make to perform an action.)
You can also find details about the Discourse API here: https://docs.discourse.org/.