String is an item from Vanilla Minecraft dropped from spiders and cave spiders upon death. It can be used to make bows, wool, fishing rods and a few additional items.
In TiC, string can be used to make silky cloth, required for the Silky modifier. It's also used to make a Bowstring, a crucial part of the Shortbow, Longbow and the Crossbow.
It's also in a crafting recipe for the book along with two blank patterns instead of a piece of leather and it's required to craft the Traveller's boots.