Sidecar is a great new feature introduced in iPadOS 13 and MacOS Catalina (10.15) that gives you the opportunity to use your iPad as a secondary display.
To enable sidecar, go to “System Preferences”, select the “Sidecar” icon and choose the device you would like to connect. You can choose to have the secondary device act as your right or left display.
After you enable Sidecar, you can even use an Apple TV as an additional display. The device that you are connected to requires the same Apple ID. If it is different, your computer will not find it.