Height Mapping is a technique to blend textures based on a height map. This is useful for materials like a uneven stony road on a grass terrain, as the materials gradually blend, the grass is first filling the spaces between the stones before the stones completely vanish.
For this to work you need a height map of the texture like here:
Diffuse, normal, specular and height map. (For the creation of this texture image see Creating_Seamless_Procedural_Textures_with_Blender#Bricks_as_a_"Quad_Texture")
Height mapping is also explained/shown here: