From MongoDB's JSON Schema Examples Tutorial page:
JSON Schema is an IETF standard providing a format for what JSON data is required for a given application and how to interact with it. Applying such standards for a JSON document lets you enforce consistency and data validity across similar JSON data.
The purpose of a JSON Schema is to define the allowed property names and data types allowed to facilitate the validation of a given JSON object. This reminds me of TypeScript definitions, where type-checking happens by default after you've instantiated a given interface or class.