A chemical extracted from plants and trees used for turning hide into leather.
Tannin is used to make Leather.
Tannin is made from Tree Bark
Currently by the tailor and tanning racks to the north of the the tutorial islands.
This is going to be craftable eventually. You're going to havest plant material and use alchemy to synthisize/extract the tannins. I need to do a bit more research for the process.