They're calling them "Shakin' Flavor Fries" and the seasonings are available for free with a large order of fries or with a small surcharge for smaller sizes. The available flavors are Ranch, Buffalo, and Garlic Parmesan.
Basically, the way it works is: you dump the fries into a bag (they're calling it the "Fry Bag"); you open the provided seasoning packet over them; you close the bag; and you shake until they're well coated. It's like Shake 'N Bake without the baking (I suppose that's arguable...).
While the concept is new for McDonald's in the U.S., it's been occasionally available in certain Asian countries (including Japan, Hong Kong, Malaysia, Singapore, the Philippines, and India) for some time. There, they've gone by names emphasizing the shaking such as "McShaker Fries" and "Shake Shake Fries." The various seasonings have included the more exotic such as piri piri (shown above), seaweed, and curry to the decidedly mundane like salt and pepper.
In Japan, they even have the same concept for chicken.
Apparently, some locations are offering the new fries for free through Friday, May 9, 2014.