Learn more about JSON Schema on the official website for that standard. JSON Schema provides a way of describing the data contained in a JSON blob so that a form can be built to modify those values. HAX uses this extensively via a tag called eco-json-schema-object and related tags, to turn any json schema definition into a fully two-way data binded form.

HAX schema is an abstraction that sits on top of JSON Schema in order to provide forms for updating values in your HAX wired elements. If you wanted to extend the types of input fields that HAX has access to then learning JSON Schema would be the best place to start!