Emacs is a Platform. Operating Systems are also Platforms. Many other things are platforms. A platform is a layer of software designed to enable you to build new things on it. This should raise the question: What platform X would be good for building this thing Y I want to build? I often find X = Emacs, but also Racket, Rust, et al. An application which is also a good platform may be worth a greater learning investment than a simpler application would justify.