It's called Astropad (http://astropad.com) and it allows you to use your iPad as a graphics pad with your Mac. So, instead of swapping back and forth between the Mac and the iPad for editing tasks – copying files back and forth – you can use them together.
You need to launch an app on both platforms and from there it's relatively seemless – changes you make on either platform are immediately echoed to the other. There's also a palm reject function on the iPad – handy for stylus users.
It's pricey, at $50 ($20 for an academic license), but if you do this stuff all day, it might be worth it. And, it's cheaper than a full blown graphics tablet.