Often written as B & B, it stands for Balled and Burlapped. It is udes to indicate a plant that does not come shipped in a pot. Rather its roots have a little bit of soil wrapped in burlap and tied together with twine or wire. It is often used as a way to ship trees and large shrubs.
When planting B & B plants, we generally recommend keeping the burlap on and removing the twine or wire. This is to ensure that any fine roots that may have grown through the burlpa are not damaged during planting.