I may be very wrong, but I think it has to do with the phases of the moon, and the effect of that on Tamriel.
A lattice is a framework.
It may be the frame that defines and encapsulates Nirn.
In my mind I always see a sort of chart, that displays the phases of the moons on it in little pictures and some mystical text below it that describes the effects on Khajit birth, alchemy and the like.
Like the periodic table of the elements describes the 'lattice' of our physical reality.
Of course this might all be gibberish.