sieves in nLab

Hello, I am confused about the notion of "encoding" regarding sieves: ." And sieves on objects are mostly used in the context of Grothendieck topologies to encode presheaves that behave like coverings" "Sieves are an equivalent way to encode subobjects of representable functors in a presheaf category in terms of the total sets of elements of such a subfunctor." "It is in this way that sieves and their associated subfunctors encode the notion of cover of an object V: they tell us which of all the maps into V do factor through the cover." Regards, Vasili [For admin and other information see: http://www.mta.ca/~cat-dist/ ]