Relay.dev is an open-source JavaScript framework developed by Meta (formerly Facebook) designed to manage and efficiently handle GraphQL data. Built to complement React applications, Relay optimizes data-fetching patterns by allowing developers to declare data dependencies directly within their React components. Relay promotes performance improvements through techniques like data caching and state management, strategically reducing the amount of data transferred between clients and servers. It supports building complex applications by enabling sophisticated data-fetching capabilities, such as pagination and mutations, while maintaining a declarative programming style.