Basic Definition

Weft is the set of threads that runs horizontally in a fabric. In denim, the weft yarns are traditionally fine and are not left dyed. Weft is responsible for the lighter color of the inside of a pair of jeans. Weft also enhances the effects of abrasion.
edit

weft diction

According to the Oxford English Dictionary, like warp, weft also has several interesting definitions aside from being the threads that cross from side to side of a web.
*weft is the cane, palm-leaf, straw, etc. used as the filling, in weaving baskets, mats, etc
*weft is used in a figurative context, Example: “The weft of the world was untorn. That is woven of the day on the night.”
*weft is also a layer of closely interwoven hyphæ produced in certain fungi
edit

See Also

warp
twill
denim
edit

References

The Denim Bible Jeans Encyclopedia II - by Sportswear International
Oxford English Dictionary http://dictionary.oed.com/
edit