A web app manifest is a small file that gives a browser details about a website when it can be installed like an app. It is like a card that tells the device the app's name, icon, and preferred colors.
The browser can use these details when adding the site to a device's home screen. The installed shortcut can then look more like other apps.
The manifest describes the app-like presentation, but it does not turn every website into a fully offline app. The site still needs its own features and behavior to work the way people expect.