I think it just means that the standard 1-quart Mason brand canning jar will fit the equipment. Basically, the mason jar becomes the container that the blade assembly fits into, and the actuall chopping/processing occurs inside the jar.
Interesting, why would one use the Mason jar instead of the container that comes with the device? I'm ignorant of food processors and a bit puzzled, because I saw Aajonus use the original plastic Cuisinart container in a video:
Fast, Nutritious, & Delicious Raw Food Recipes DVD SamplerI realize that people north of the Mason-Dixon line don't always know about Mason jars, etc.. 
I literally have 5 or 10 in my (redneck) kitchen right now, though. It's a very useful little 1-quart glass jar.
Huh?
Them's fightin' words!
I've always thought of the Mason jar as part of frugal Yankee culture and I'm a history buff, so I looked it up. The Mason jar was invented shortly before the Civil War by a New Jersey-born man living in Philadelphia at the time. It was Yankee factories in Camden, NJ and other Yankee cities that made mass production of Mason jars possible and that were also crucial in Yankee victory in the Civil War. Mass production food preservation methods involving Mason jars and other inventions like tin canning revolutionized the food industry (much to the detriment of long-term health, unfortunately). The Union used Mason jars in feeding its enormous armed forces that subdued Southern secessionists. The immigrants who flocked to the North to work in the factories that made things like Mason jars also helped fill the ranks of the Union Army.
My mother read about ways to use Mason jars in another Yankee institution--Yankee Magazine, which uses the narrower definition of Yankee as New Englander, with a focus on the culture passed on from early English settlers of New England. So the Mason jar is indeed a Yankee (Northern) cultural artifact popular with the famously frugal traditional New England Yankees. If I hadn't heard about Apalachian Southerners drinking moonshine from Mason jars, I'd be surprised that people
south of the Mason-Dixon line are familiar with the Mason jar or don't despise it as a symbol of Yankee craftiness and oppression.
It's the food processor that I'm unfamiliar with and that's foreign to traditional Yankee culture. My parents and I have never owned one and my mother has spoken derisively of it, but I can see some potential uses for it now. My youngest sister is more new-fangled. I think she was the first in the family to get one, though I've only seen her use it on one or two occasions, I think.
Of course, it's common for people to acquire the inventions of foreign divils and make them their own, so I don't at all mind that using the Yankee Mason jar has also become a cherished part of "redneck" Southern culture. So if you should take a drink of moonshine/mountain dew/white lightning/poitín from an old Mason jar, be sure to toast the Yankee who invented the jar.
