What exactly is bullous emphysema?

Holes, large: Bullae is a medical term referring to a bubble like structure; bullish emphysema is advanced disease with large air sacs in the lung where normal lung tissue has been destroyed.

Big spaces: Emphasema comes in a variety of different subtypes - in bullous emphysema, lung tissue is destroyed and leaves behind large spaces that don't contribute to gas exchange. Sometimes these spaces can get infected (bullitis) or they can press down on healthy lung preventing it from functioning (removing these is the basis for lung volume reduction surgery as a therapy in copd).