Though they're often called "nuts," these tubers are actually the root from the chufa sedge plant. They were originally cultivated in ancient Egypt, but today are common in Southern Europe too, particularly in Spain.
Tiger nuts are often soaked in warm water before being eaten, and they have a sweet, nutty flavor. In Spain they are used to make horchata, a sugary, milky drink. In fact, it can make for a good milk substitute for those who are lactose intolerant or vegan.