Web browser push notifications are messages sent to a user's web browser, such as Chrome, Firefox, Safari, or Opera, from a website or web application, appearing as small pop-up notifications even if the user is not currently on the site. Unlike mobile push notifications, which are delivered through apps using technologies like Firebase Cloud Messaging or APNS, web push notifications rely on a W3C standard implemented by browser manufacturers. The tutorial discussed in the text demonstrates a web application named "Oi," which uses web push notifications to send messages between browsers in under a tenth of a second, featuring real-time presence functionality that indicates which users are available to receive notifications. It utilizes the PubNub JavaScript SDK for Pub/Sub messaging, ensuring that users can send and receive notifications efficiently. To implement web push notifications, developers must first request user permission, and once granted, they can display notifications directly using the Notification API. Web push notifications are commonly used by services like Gmail and online calendars to alert users of new emails or upcoming appointments, respectively.