Delicately collected strands of plant. Perfect for making thread, cloth, or rope.
— In-game Description

Fiber is a natural resource that can be gathered from plants everywhere (e.g. berry bushes) by hand, using the Metal Sickle, or a creature such as Therizinosaurus, Gigantopithecus, or a Direbear. Fiber is used for a variety of applications, including Thatch and Wood construction, tools, weapons, and armor.