Cookie warnings

I got a little bag of cookies today with lunch (ack! what happened to my diet?!). I was for some reason glancing over the ingredients, and noticed the following warning:

MAY CONTAIN TRACES OF TREE NUTS.

What the heck is a tree nut? Is it anything like a tree squirrel? And why do I picture it like a peanut-guy, perched in the crook of a tree, wearing a propeller beanie?