Deep link is a URL that opens one screen inside an app — a reservation, a product, a class — instead of the home screen. The person taps once and lands on the thing you meant, even if the app was closed.
A push notification is the alert, and the deep link is where the tap goes. The app icon still opens the front door. A normal website link opens the browser, unless the app is set to claim that address.
In a React Native app, navigation has to know the route or the tap dumps people on the home screen. A native app does the same job with the phone’s own linking. A web app already is a URL; a deep link is that address inside the installed app, on a step of the user flow.
A real-life example of a deep link
An Atlanta dog walker texts a client that Saturday’s walk near Piedmont Park is confirmed. The link opens that booking in the app, with the time and the park, not a blank home screen. A link that only launches the app and makes them hunt for the walk is how people call the shop instead.
Point the tap at the screen that answers the question. See navigation in React Native and opening a screen from a push notification.