GraphQL is a query language for APIs and a runtime for executing those queries with your existing data. Coursera's GraphQL catalogue teaches you how to use GraphQL to replace or augment existing REST APIs, providing increased efficiency and power to your applications. You'll learn about the basics of creating a GraphQL server, designing and implementing types and schemas, handling queries and mutations, and integrating GraphQL with popular libraries and frameworks. By the end of your study, you'll be able to build flexible and performant APIs that can be tailored to the exact needs of your clients, providing a powerful tool for modern web development.