Terrain files are textures, in the format of BLP obviously.
You import them and change its path ( Double -Click the file, check (Use Custom Path) then Put one of the paths of the link above that Veritas attached )
For example :
TerrainArt/LordaeronSummer/Lords_Grass.blp
it's to replace Lordaeron grass texture...
If you import a terrain cliff texture. Change path to either of this :
ReplaceableTextures/Cliff/Cliff1.blp
or
ReplaceableTextures/Cliff/Cliff0.blp