Manioc, or cassava, has a few uses. It looks similar to an elongated potato and grows underground (hence the root). In modern times it is currently being used for animal feed. As people continue to look for alternative fuel sources, manioc is lending its help. Manioc can be used to produce ethanol and be added to existing petrol. Lastly, manioc can be ground down and turned into a flour. That flour could be used for things like baking bread.
I am assuming you would like to know how the root was used in the book The Swiss Family Robinson. The family used it as a food source. They ground it up and used it as a flour substitute in order to make bread and other foods while stranded.
"If this proves to be, as I expect, the manioc root, we might lose every other eatable we possess, and yet not starve. In the West Indies, cakes called cassava bread are made from it; and, already having potatoes, we shall be very independent if we can succeed in preparing flour from these roots."