There's a lot you can do with a tablet, but not enough that it's worth using instead of the laptop or desktop that's sitting in front of you. So why couldn't you use that big-ass device as an extra display? Third party apps have supported it for a while, but now Apple's doing it officially.
It's called Sidecar (geddit) and means you can hookup your iPad to your Mac running the new Catalina OS and use it as a secondary display. Wirelessly, it seems, according to the promo shots Apple showed off on stage.
Apple also revealed that this will be supported by app developers that already have dedicated iPad apps. Like Adobe.