What popular food product beginning with the letter t is obtained from a highly poisonous root that locals call manioc?

The popular food product you're referring to is tapioca. It is derived from cassava, a plant whose root is highly toxic when raw due to the presence of cyanogenic glycosides. After proper processing, which involves peeling, soaking, and cooking, the toxic compounds are removed, making tapioca safe for consumption. It is commonly used in various dishes, including puddings and bubble tea.