A curtain was a fabric covering that concealed one's view. They often hung at windows, but could also be used to partition one section of a room from another for privacy. Some curtains hung on rods by multiple rings.
In 2376, when Neelix told Tuvok that he was haunted by the nebula outside the windows of USS Voyager's mess hall, the Vulcan suggested Captain Janeway might give him permission to install curtains. Neelix seemed pleased with the idea, adding that he knew exactly the material to use. (VOY: "The Haunting of Deck Twelve")