(That's right: 'Kerley B', not 'curly bee'! The reason for the silly name: Kerley was unfortunately someone's surname, and he identified three types of lines on chest X-ray, which he called A, B and C. B is the most famous, and you can basically forget about the other two.)
Kerley B lines are an X-ray finding that represents visible interlobular septa in the lung. As implied, these septa are usually invisible on X-ray, but certain conditions can fill them with radiopaque stuff.
The most common condition causing Kerley B lines is cardiac failure. In this case, the backing up of fluid in the pulmonary vasculature eventually breaches the blood vessel wall and floods the interstitium, of which the interlobular septa are a part. Alternatively, other conditions may fill the interlobular septa with different material, like fibrous tissue (in pulmonary fibrosis), cancer cells and their attendant infiltrate (e.g. carcinomatosis) or even haemosiderin (from recurrent pulmonary oedemas in which red cells made it to the interlobular septa, only to die there).
Kerley B lines are best appreciated in the lower lobes, where they are seen as transverse lines near the lung peripheries. Even at the best of times they are hard to see, however - as this supposedly 'classic' image shows. (The white arrows point to some of them, but if you are seeing any transverse white lines in the area you are probably looking at them!)