GraphQL is a query language for APIs that allows developers to define the structure of their data and request only what they need, making it more efficient than traditional REST APIs. The GraphQL API at Contentful uses abstract syntax trees (ASTs) to represent schema definitions and parse queries, providing a rich ecosystem for development and tools like GraphiQL for exploration. The content infrastructure at Contentful allows users to define the structure of their content freely, making support for GraphQL more complex, but enabling features like schema stitching, which combines multiple APIs into a single endpoint. The GraphQL API is designed to be strongly-typed, with type definitions defining how data should look and what queries are allowed, providing a new level of power in API development.