GraphQL is a query language and server-side runtime that serves data over HTTP, aiming to improve the way developers build applications on the web by providing a more efficient, performant, and developer-friendly approach to client-server communication. It was designed to reduce complexity in app development by allowing developers to fetch only the required data from various sources, reducing bandwidth consumption and improving performance. GraphQL has gained significant adoption since its open-sourcing in 2015, offering benefits such as strongly typed APIs, hierarchical and declarative data querying, and decoupling of clients from servers, making it a popular choice among notable companies like GitHub, PayPal, and Twitter.