Astro HQ’s Luna Display dongle for the Mac turns an iPad into a secondary monitor via Lightning or Wi-Fi. Others have pulled off such an iPad-as-Mac-screen maneuver using software, but Luna’s hardware approach taps into the Mac’s graphics acceleration to improve performance.
If you rely on a third-party solution like AirDisplay, DisplayLink, Duet, or iDisplay for extending your display to a second screen, hold off on upgrading to macOS 10.13.4 High Sierra, since it currently breaks those products.