It's where people go and knit around things at public places, like fire hydrants, trees, mailboxes, bicycles, and even whole cars and buses have been yarn bombed.

I want to learn how to knit now, and go yarn-graffiti the town. There have been yarn "gangs" too, being Knitta Please, YarnCore and Knit the City.
While it's still counted as illegal vandalism and/or littering, they say that it's more likely the cops will laugh at you rather than give you a ticket or do anything serious.
I wanna' go Yarn Bombing, guys.