The idea is that whenever you have a server (which can be a remote server, but also just another process in the same machine, or even a sandboxed WASM module running in the same process as the renderer) they can send some simple messages and those can be turned into an interactive and reactive UI.

The renderer can be running in a terminal, a native app or basically anywhere else.

I guess it's somewhat like HTML, but purposefully much simpler so it can be implemented very easily, with built-in reactivity, no arbitrary code execution and a very limited set of components, so the server (mostly) can't influence _how_ things look on the client, only _what_.