By default, whenever you install a mobile app, you give the app the permission to send you push notifications on your device. Websites, however, have to explicitly take permission from their users to send them push messages.
This is how website push notifications work: The first step is getting opt-in from visitors. As soon as someone arrives on a website, an opt-in box is triggered. If the visitor clicks on “Allow”, he/she is added to your subscriber list.