GHCJS is a compiler that allows developers to write web applications in the Haskell programming language and then convert that code into JavaScript. This enables Haskell developers to create interactive web applications while leveraging the strong type system and functional programming features of Haskell. By using GHCJS, developers can take advantage of existing Haskell libraries and tools, making it easier to build complex applications. The resulting JavaScript code can run in any modern web browser, allowing for a seamless user experience across different platforms.