I am with you. It is hard to tell what it is. After reading the docs it seems like they have a backend (Foundry Ontology). To make development for it easier, you can now generate an SDK (Ontology SDK) that is specific to your project (or your project's ontology) using their Developer Console. You can use this SDK with your favorite language, like Typescript or Python.

It took me too long to figure it out to even consider developing for it.