Notifications are interactive alerts from apps and macOS that help keep you informed. For example, when you receive a FaceTime call or an invitation in Calendar, or when a software update is available, a notification appears in the top-right corner of the screen. Many notifications let you respond or take other actions directly in the notification.
You can customise notifications or turn them off in System Preferences. To see notifications in Notification Centre, click the date and time in the menu bar at the top of the screen.