Ghostty is a terminal like iTerm. This compiles it so it runs in the browser directly, or browser-based environments like VS Code or the Hyper terminal. Without that you’d have to reimplement a whole terminal in JavaScript. Which is what people have been doing with via the xterm.js project. Naturally, there is effort and bugs that go into maintaining a clone/port like that. This lets you use the Ghostty terminal code directly - compiled to WebAssembly and with no other dependencies - as an API-compatible drop-in replacement

Only other relevant thing to add is that Ghostty is also written in zig and makes for a good showcase of the language.