ATA Carnet

It sounds like a fancy red wine ("Ask the sommelier if the 1994 Ata Carnet will pair appropriately with the braised veal shanks in rosemary cream sauce"). But, in fact, it's a document used in certain customs transactions.
Sometimes a product is imported into a country with the intention that it will eventually get exported somewhere else. Basically, it's coming for a visit, not to live there. A carnet allows this process to happen without the payment of import duties or taxes.
It's sometimes called a "merchandise passport," allowing the item to pass through multiple countries on its way to its final destination.
Imagine taking a tanker truck full of maple syrup from Canada to Mexico. You have to drive through the U.S., but the syrup isn't staying in the U.S. and it isn't getting sold in the U.S. You would obtain an ATA carnet for the shipment. Then you can drive the truck through the U.S. to Mexico without paying U.S. duties, though you'd eventually have to pay whatever fees are required by the Mexican authorities.
Carnet is the general term for this type of document. The ATA part designates a group of 87 countries that belong to a particular carnet system. Various countries may have other carnet systems with different specific regulations. For instance, the U.S. has a separate carnet with Taiwan known as the TECRO/AIT carnet, which has a different set of rules from the ATA Carnet.