Tannins are natural botanical dyestuffs that possess a high quantity if tannins. There are several different tannin groups. Tannins bond to fibres and can be used as dyes in themselves and also to assist other dyes and mordants to bond to fibres. Tannins are recommended for use with cellulose fibres as a pre treatment when mordanting with Alum. See here for more info on mordanting here. 

Tannins will also have a strong reaction with Ferrous (Iron) mordants and can produce dark tones, greys and blacks.