The project is reportedly codenamed "Sidecar." While it's already possible to use an iPad as an external display for Macs, thanks to paid-for third-party solutions such as Luna Display, the aim here is for Apple to integrate the function directly into its operating system.
This would make it possible to view any application window open on your Mac directly on your iPad. The function promises to be extremely simple to use, with a new menu proposing the option to move a window to an external display or iPad appearing when the user hovers over the green "maximize" button at the top left of an app window.
If the iPad in question supports Apple Pencil, it will apparently be possible to draw or make annotations directly via the tablet's touch screen.
The major new features of macOS 10.15 are expected to be outlined at the Apple Worldwide Developers Conference (WWDC), held June 3-7, 2019, at the McEnery Convention Center in San José, California.