A notarized app is a macOS app that was uploaded to Apple for processing before it was distributed. When you export a notarized app from Xcode, it code signs the app with a Developer ID certificate and staples a ticket from Apple to the app. The ticket confirms that you previously uploaded the app to Apple.
On macOS 10.14 and later, the user can launch notarized apps when Gatekeeper is enabled. When the user first launches a notarized app, Gatekeeper looks for the app’s ticket online. If the user is offline, Gatekeeper looks for the ticket that was stapled to the app.