Push notifications are a type of technology that allows an application or a service to send messages or alerts to a user's device without requiring the user to open the application or the service. They can be used for multi-factor authentication (MFA) by sending a prompt or a code to the user's device that the user has to approve or enter to verify their identity. They can be non-disruptive and user friendly because they do not require the user to remember or type anything, and they can be delivered instantly and securely.