There are a few popular apps on the App Store that allow you to connect your iPad to your Mac as a secondary display. Ideal for expanding your workspace, especially when using the 12.9" iPad Pro. Duet and Air Display are both great options, but they have quite a few limitations. Luna Display, from the makers of AstroPad, aims to be the end-all solution for a lag-free, HiDPI experience.
To see it for yourself, check out our hands on video with a prototype of the Luna Display.