Has to do with the arrangement of the atoms in the material forming elements that repeat in a three dimensional lattice to form a crystalline structure. Or something like that. Or, could be as simple a early piezos were quartz crystals.

Remember something like this in physics when studying semiconductors and silicon structures (1,0,0),(0,0,1), (0,1,0) Miller Indices, etc.. Now my head hurts thinking about it..

-danny