Tapioca Pearls, also pearl tapioca, are small spheres of tapioca starch (made from cassava), and are the basis of many puddings in Asia and, through colonialism, parts of Europe. They are also used to make a drink called bubble tea.
Tapioca pearls normally require soaking for some hours before cooking, although some commercial brands are prepared for instant use (i.e. without soaking); read the packet before use.
Tapioca pearls are often used as a substitute for sago. The practice is so common that some brands of tapioca pearls called themselves sago, but closer inspection of the packet reveals that they are actually made from tapioca.