GraphQL is a query language for APIs that allows clients to request only the data they need from APIs, making it more efficient and flexible than REST. It was developed at Facebook and has gained popularity among developers and companies building supporting tools and GraphQL APIs. GraphQL is language-agnostic, allowing it to be built in various programming languages such as Python, JavaScript, Java, Scala, and others. The GraphQL schema defines the types of data and operations allowed on that data, and functions for the fields on the data types. It has its own language, the GraphQL Schema Definition Language (SDL), which is used to write GraphQL schemas. GraphQL queries are used to fetch data from an API, while mutations are used to create, update, and delete data in a database. Resolvers are functions that return values for different fields defined in the schema. Ariadne is a popular Python library for building GraphQL servers, and it uses a schema-first approach, where the schema written in SDL is the ultimate source of truth. The tutorial covers how to build a basic GraphQL server using Flask and Ariadne, including creating queries, mutations, writing a schema, implementing resolvers, and testing the API with the GraphQL Playground.