others can tell you more, but the pawnstructures have to do with it.
If the pawns are on white squares the black squared bisshop (+queen) can move freely through them = strong.
However, I have had end games in which I it was the other way around. The same square bishop was able to eat up my pawns..
Black's got some weak dark squares:
pawns on light squares = dark square weaknesses. however if a dark square bishop is still on the board to watch over those dark squares, they are normally not an issue. and vice-versa. this is why it is common to see one side try to swap off the defending fianchetto'd bishop to weaken squares around the opposing king.
Of course, they're only weak if Black will be able to take advantage...
Bronstein adds an extra element that I don't see often explained. The following is from his introduction to game 1 (Szabo-Geller, Zurich 1953) of his famous Zurich International Chess Tournament 1953. I give the game without commentary below the quote.
I have long suspected, whenever the books I have read began discussing darksquare weaknesses or an attack on the dark squares, that the subject under discussion was not only beyond my understanding, but beyond the author's as well. "Certainly," I would say to myself, "it must be true that the enemy dark squares will be weak if his pawns stand on light squares and he loses his dark-square bishop. But if he then removes all of his pieces from the dark squares, what will be left for me to attack?"
Such was my line of reasoning, until the day I realized that a weakness of the dark squares is also a weakness of the pieces and pawns on the light squares. Light-square weaknesses are also possible, resulting in a weakening of the enemy pieces and pawns on the dark squares -- as occurred, for example, in the Geller-Najdorf game in Round 13. The point of an attack on the dark squares is that, by placing my pawns and pieces on the dark, I attack my opponent's pieces and pawns on the light.
The Szabo-Geller game provides a clear example of the method of exploiting a darksquare weakness; and the combination which was possible after Black's 24th fairly begs to be included in a textbook, taking place as it does entirely on the light squares.
Yeah, I always think of that Bronstein thing too... 
Yeah, it made a lot more sense to me after reading that and going through that game carefully, and it's the first thing that comes to mind for me now when I hear somebody puzzled about the idea.
Dear all,
I often see this idea. Can you explain it to me? What are they? How do you spot them?