Garri is made by processing cassava, typically through grating, fermentation, pressing and roasting into dry granules. It can be prepared with hot water as eba or used in other dishes and drinks according to regional practice. White and yellow varieties are common.
It is not raw cassava flour and should not be replaced with that flour without changing the recipe. The spelling gari can also mean Japanese pickled ginger; see gari for the distinction.
Reference: Further reading.