Cellulose is an organic compound found in trees and plants. It is most often used to make paper. Cotton is 90% cellulose.
Cellulose can also be broken down and made into viscose fibers that are spun into yarn. Rayon, Tencel, and bamboo yarns are all made this way.
All of these yarns are made from plant material and are renewable resources. Often (but not always) they are made in very eco-friendly ways.