Arrowroot, or arrowroot starch, is a powdery product made from the arrowroot plant, a starchy tropical root. The plant is dried and ground up to make the powder, which is primarily used as a thickener in the kitchen, much like cornstarch. Much like cornstarch, it is an excellent thickening agent. It has about twice the thickening power of regular flour. Unlike cornstarch, however, arrowroot is completely flavorless and will not impart a starchy taste into puddings or other dishes that it is used to thicken.
Another reason that arrowroot is called for in recipes is that it is extremely digestible, moreso than regular wheat flour. It can be used in cookies and crackers as a substitute for part of the flour, and you can find “arrowroot biscuits” in stores that are often said to sooth the stomach. It is also a very popular ingredient in gluten-free baking. Still, it’s primary use is as a thickener. To use it, it must be mixed with some cold water before being added to a hot liquid to prevent clumping. Arrowroot can be found at some specialty grocers and can also be ordered online.