When you create an event in Google Calendar, you have multiple options for how you want to be reminded. Google can shoot you an email or text, though if you know you will be at your desktop, a pop-up notification may be better. But those things can also be pretty intrusive.
Google has updated the web version of Calendar to make those pop-up reminders less in-your-face. Instead of a static window appearing in the middle of your screen, an alert slides in from the top right corner. The card is slick, modern, and contains relevant details.
There isn't much else to it than that. Okay then, back to work.
These notifications are opt-in and work in any browser that supports the web notifications API. Users who already enabled this as an experimental lab feature will see it stay on automatically.