It‘s a progressive web app, which is a little different than a website; it’s built in HTML/JS like a website, but has no server. You can install it on your device and it talks directly to your Lemmy instance. In effect it’s no different from any other Lemmy app except for being built in web programming languages instead of Swift/Kotlin (app programming languages).

Open source, so anyone can look through the code, or self host it even.