Platonymous submitted a new mod: More Map Layers - http://www.nexusmods.com/stardewvalley/mods/1134/ Read more about this mod...
On it's own: Absolutely nothing. But if you creates a new map, or edit an old one, you can put new layers between the "Back" and "Buildings" layer in your map editor and it will draw them. Without the mod they would be ignored by the game. There's no mod that's actually using this yet, but I needed it for a mod I'm working on and so I decided to make it available for anyone.
It's not missing, I just checked to make sure. Either you're download didn't work or you didn't extract it correctly.
Well that's... odd. It worked once I redownloaded it, guess the download corrupted on my end or something? In any case I'm glad its working, sorry about the trouble