It's a pumping lung in a box, basically. Al Jazeera America specifies that its more properly known as the Organ Care System (OCS) but it's basically a human donor long being kept alive and breathing out of the body inside a box. The OCS machine is used to keep the blood and oxygen flowing to the donor organ so that it can buy itself more time before the donor organ is given to the recipient.
What's especially neat about the OCS is that they can actually be used to improve imperfect donor lungs by flushing it with antibiotics and nutrients. Like refurbishing a lung, sort of. Putting donor lungs through the OCS helps increase and improve the number of potential donor lungs. Not every donor lung is usable, donor lungs that go through the OCS may be.
Shini Somara of Al Jazeera America reports on the lung transplant:
The sound of the lungs mechanically breathing was surreal, and we followed these previous organs back to the OR. Once surgeons were ready to make the swap, they held Victoria's old lung and new lung side by side. The difference was night and day. Victoria's old lung was tiny frail and full of ugly black carbon deposits, while her new donor lung was large, pink and fluffy.
You can follow the whole lung transplant story and more details about the OCS over at Al Jazeera America.