Burl is basically an malformation, on a tree, a natural anomaly. While still in the tree, it kinda looks like an outie belly button, for lack of a better way to describe it. The burr, (belly button) is a section of the tree that has not grown properly, and is usually a cluster of small knots and or buds. Burl can also be derived from the roots of a tree. It’s said to be caused by stresses, and disease or bugs.