Masks set the ground textures in your map. For example, grass on an island, sand inside the rivers and rock up in the mountains. They have the same size as the heightmap, but don't have as many required settings as them. You paint on it with pure red, green and blue (RGB 255/0/0, 0/255/0 and 0/0/255) to show the game where your textures should be. You can use the heightmap as reference where the textures should be. When you are finished, type the name of the file in the box below the box you typed the heightmap in.