it goes along with phrases such as: the light's on but nobody's home, not the sharpest tool in the shed, not the brightest crayon in the box, etc. basically just means whoever he is talking about isn't very smart.
Ha! I've never heard that one! It just means the person you're talking about is not very bright. You know, like a few bricks short of a full load, a few fries short of a Happy Meal, not the sharpest knife in the drawer, and the list goes on!