The Old Root is a component added by Roots. It has a 1/40 chance of being dropped when Grass is broken. It is used for various spells. It also can be eaten.
Runic Tablet entry
Old roots can be found at a 1/40 chance when breaking tall grass. They are needed for a lot of early magic, as they are required for spells without modifiers. They can also be eaten to restore a very small amount of hunger.