The Golden Horn ("Halic" in turkish) is a narrow gulf, sort of a "branch" of Bosphorus. It separates the two parts of the European side of Istanbul.
During the years of the Byzantine empire, when Constantinople was under siege, a huge chain was blocking the entrance to the Golden Horn. One of the crucial moments in the conquest of Constantinople, was when Mehmet the Conquerer, decided to drag his ships over the land and "behind" the chain. The one edge of the chain was probably very close to the spot where you see the small boats in the foreground.
The mosque you see on the other side, is Suleymaniye Camii, one of the most beautiful of the world.