The histogram reflects the fact that there's data in the "medium" and "light" areas of the histogram graph, but no data in the "dark" area of the spectrum. Since the black part of the graph is running off the right-side, you can tell that some data was lost due to over-exposure. (You can tell because the data on the graph runs off of the right-side). In this case, though, the over-exposed part where the detail was lost was the background, and the over exposure was intentional.