Tapioca flour comes from the root of the cassava plant. It basically the same thing as tapioca pearls, like you would use for pudding, but tapioca flour has been ground into a a flour.
Tapioca flour/starch adds structure to gluten free baking. It also helps give things a chewy and/or crisp texture, especially in things like cookies and cakes.
When over used in a recipe though tapioca flour can make food slimy and can also add a strong taste to the final product. Most recipes need some tapioca starch, but not too much, and finding that balance can sometimes be hard.
Tapioca flour can also be used as a thickener in sauces and gravy, but it is not my favorite thickener to use because I have found that it tends to make the sauce a bit slimy.
Recipes Using Tapioca Flour see Here