A specular map is used to allow some parts of a texture to reflect light more intensely than other parts. Say you have this barrier texture (the diffuse map):
This would be a suitable specular map:
It doesn't matter that the specular map is not the same size as the diffuse map. The specular map will either repeat more frequently or will be stretched (depending on the material settings for each map). Parts of the diffuse map that line up with the bright pixels of the specular map will reflect a lot of light, whereas the reverse is true for parts of the diffuse map that line up with the dark pixels.