I have not heard of this pastime until a week ago at a street festival.
From what I read, it is a way of displaying your yarn artwork publicly on tree trunks, light poles, or wherever your imagination takes you.
You can read about yarn bombing here.
There is a debate of whether this is legal. What do you think?
Some consider it a form of graffiti.
I like it. I think it adds color and fun to bland looking objects.
Let me know if you see any such artwork in your area.