Tannin is a resource used in for example, making leather armor. There are 2 types. Hide Tannin and Leather Tannin. Hide tannin uses bark + water. Leather tannin uses sap + water. Beginner and journeyman formulas can be purchased from gatherer trainers (and others like tailor and outfitter) with lore tokens. The tools you need are: harvest knife for bark, bucket for water, sap spigot for sap, and brew stirrer for tannin. The machine you use is a distiller. Cargo discs help a lot. I usually fill up a cargo disc with either bark or sap (bark for first half of tier and sap for second half) then fill my backpack with water and go over to a distiller and craft away. Tannin sells well to pawnbrokers. NT is a good place to start for T1 with the cedar trees, a well and a distiller nearby each other. Right now I am working on T2 so I go to Scorpion Island or Abandoned Island for elm then haul my cargo disc to a plot community that has a well and journeyman or expert confectioner shop (for the bonus to skill). I use the map mod for finding resources.