There are a total of 8 Map Stones in the game, each Map Stone is marked on the Map once you have seen it. In order to restore it you need to find a Golden Map Piece and then use that piece on the Map Stone. This reveals the Map for that area, the first one is right near the start of the game. Once the Map Stone is Restored your Map will update with grey unexplored areas for that map section and the achievement should pop.
There are some issues with tracking on these so see the below solution to find a fix if you run into this.