Sidecar is a feature that Apple touted at WWDC19 and is designed to let you use your iPad as a secondary display for your Mac
How to Activate Sidecar
Using Sidecar requires a compatible Mac running macOS Catalina and a compatible iPad running iOS 13 that have pencil support, you can view a list of these here. There are multiple ways to activate Sidecar, all of which can be done from Catalina.
The easiest way to get to Sidecar is to use the AirPlay interface on the Mac. Click the AirPlay icon at the top of the Menu bar, if you have an iPad that’s compatible with Sidecar, it will show up in the AirPlay list.
Choose the iPad that you want to connect to and it will automatically turn on and activate as a secondary Mac display. Another way you can activate Sidecar is by clicking and holding the green window expansion button on any Mac app, and you can choose the Sidecar section in System Preferences.
Sidecar is designed as a secondary Mac display, so it works like any other secondary display you might use with your Mac. You can drag windows from the Mac to the iPad and vice versa, and interact with both using your Mac’s trackpad. If you have an Apple Pencil you can use this for clicking, selecting, and other on-screen control tasks. You can draw right in apps such as Photoshop and use the iPad as a graphics tablet for your Mac