Rule explanation:
For green countryside tiles, you can’t create chains of single tiles. So, in this image, the areas marked X can’t be explored until the area marked O is filled in. In other words, a tile can only be placed next to a tile that is itself adjacent to 2+ tiles. As I mentioned above, brown core tiles are stricter: can only be placed adjacent to 2+ tiles, and cannot be on the coast (i.e., they can only be on the darkest shading on the map image).