<link rel="manifest">
Points at the manifest that teaches the page to behave like an app.
It links the document to a JSON file — the web app manifest — holding name, icons, colors and display mode. It is the piece that makes the page installable: with a manifest, it earns its own icon, a window without an address bar and a place in the app drawer.
The manifest alone installs nothing: each browser adds its own criteria — HTTPS, icons at minimum sizes, sometimes a service worker — before offering installation. For an ordinary page it remains optional and harmless; for an app, it is the birth certificate.
<link rel="manifest" href="/manifest.webmanifest">