Cellulose is a polysaccharide (a macromolecule composed of long chains of sugar molecules) made up of glucose molecules linked together with beta-1-4 bonds. As a structural part in many plants it is important for their mechanical properties. Wood and straw, rich in cellulose, are widely used on Earth for construction work, also as a composite material. It might be useful in a Martian settlement, as well. Lignin is another key element in plant structure, that serves, among other purposes, to give rigidity to woods.
The main industrial use of cellulose is for paper production. This could be done on Mars as well. Lignin is removed from the wood pulp on Earth and burned to power the paper making process. Other uses for lignin might be found on Mars as it contains significant amounts of energy.
Cotton, linen, and synthetic fibers such as Rayon are mostly cellulose. Clothing produced on Mars will likely be made from some form of cellulose fiber.