Literally, it means to take a course opposite the apparent motion of the sun viewed from the Northern Hemisphere.
Widdershins is cognate with the German widersinnig, i.e., "against" + "sense".
The opposite of widdershins is deosil, or sunwise, meaning "clockwise".
I learned this term from Terry Pratchett a few years ago as the disc in discworld rotates and the four cardinal directions are rimward, hubward, turnwise and widdershins. Instead of north south east west.
A surprising number of things on the Disc that appear to be made up fantasy are actually based on real world words, concepts, cultures, and especially bits of folklore. Sometimes messed with a bit, but oftentimes played completely straight. And it makes the jokes even better when you look it up and figure out what Pratchett was referencing when he wrote it. Some of it is quite clever.
It's true that the Discworld is a world, and a mirror of worlds (i.e. ours).
This is a big part of why I love them so much. Hardly a week goes by that I don't think of the "boots theory of socio-economic unfairness" from Men at Arms!