The haskell platform is a suite of tools and libraries that contain the most important and best supported components. it is meant to be a starting point for haskell developers who are looking for libraries to use.
installing this meta package will also install the debian packages containing the libraries and tools as specified in the official haskell platform, in the version available in debian. this may or may not be the version specified in the platform. if this is of relevance to you, please check them on the official haskell platform homepage.