It is a piece of plain thread that can be used in building and crafting. It cannot be used as a whip such as vines can, despite being of a similar size and shape. Cords are not crafted through combinations, but are made when you take a piece of Cotton/Wool and Use it in the inventory. It's very useful to have in survival, but seeing as how one source of the base material is a unit and the other is an easily destroyed object, you should take care to make sure that you have a good steady supply. Making cords en masse can be time-consuming due to the timed "using" process, so try not to make it when you're short on time.