QuickScreen mirrors your phone‘s screen quickly to your Mac.
Even though the user interface is super-simple (and quick!), there are a couple of things you can try if your phone does not connect somehow:
Launch QuickScreen on your Mac. Make sure your iPhone is connected with a lightning USB cable to your computer. Newer iPads are connected with a USB-C cable. Make sure that you allow your computer to access your phone (there might be a pop up on your phone to “trust this computer”).