It looks like this year might be a big one for Apple's software, as rumors keep cropping up in regards to the upcoming versions of its operating systems for different devices. A new report from 9to5Mac suggests that macOS 10.15, which is expected to arrive later this year, will allow users to display their apps on an iPad screen through a wireless connection.
The feature seems to be inspired by Luna Display, a dongle made by a company called Astro HQ, which lets you use your iPad as a second screen for your Mac. According to the report, the feature is internally referred to as "Sidecar", and it doesn't seem like it will be a second display in the sense you might expect.
Essentially, when you hover over the "Maximize" button on an app's window, you'll be presented with a list of options, allowing you to send that window to an external display, where it will open in fullscreen. When using the iPad as a display, you can even use the Apple Pencil to draw, making the iPad serve as a drawing tablet for your computer.
This feature seems to be most useful for graphic artists, but it could also just make multitasking easier. Another improvement to macOS in this regard is the ability to snap apps to either side of the screen, as Windows 10 currently supports.
macOS has already been rumored to be splitting iTunes features into separate apps, all the while iOS is getting its own set of improvements, some of them focused on the iPad. These include windowed apps as well, so it would make sense that macOS apps can be windowed on the iPad screen, but that remains to be seen.