MongoDB is a NoSQL document database that stores its documents in a JSON-like format with schema. GraphQL is a query language for APIs originally developed for internal use in Facebook before being made open source.
GraphQL means you can create a query that works on and returns only the fields you want from an API, similar to a MongoDB projection or a SQL query. This is much simpler than making an HTTP request to an API endpoint and receiving the full payload of that resource as a response.
The Stitch GraphQL API lets client apps access data stored in a linked MongoDB cluster using any standard GraphQL client. Stitch automatically creates GraphQL types for every linked collection that has a defined document schema and evaluates role-based permissions for all GraphQL requests.