We've been waiting for this one.
In a nutshell, the mod allows you to build the unique buildings that populate the Sanctuary Hills settlement. Oddly enough, you can't do this in the vanilla version of the game.
Once you get the mod added you'll notice a new addition to your Workshop. Simply head to Structures > Metal and you'll see Pre War Sanctuary & Post War Sanctuary.
The tool allows you to snap together buildings using floors, walls, railings, and roofs. All of them look fantastic.
There are also several color options to choose from giving you a nice variety for your rebuild in Sanctuary rebuild or another part of the Commonwealth.