Arrowroot is a starch that is obtained from tropical plants.
It thickens at a lower temperature than flour or cornstarch and is not weakened by acidic ingredients like cornstarch can be. It has a more neutral taste, and is not affected by freezing. It does not mix well with dairy though as it can form a slimy mixture.
Overheating tends to break down arrowroot’s thickening property so I prefer to use arrowroot in pies and strudel where lower temperatures are occurring throughout the process rather than the high heat boiling of a custard recipe.
Two teaspoons of arrowroot can be substituted for one tablespoon of cornstarch.