These are a famous stop on the tours of the Medina in Fez, and worth seeing.
The shop owners will give you a sprig of mint to hold under your nose....take it....and then take you to the roof to see the "back room", where the dyeing takes place in these vats. The hides are dried and cured on the roofs, and then men, knee deep in the pits of "pigeon poop", and natural colors, dye the skins to be made into various lovely leather products.
The smell is terrible, but the leather products are lovely and very inexpensive. Each product has it's own area in the medina and if you ask for the leather center, you will be pointed in the right direction.