Idli rice is an Indian rice which is parboiled. Parboiled rice is pre-processed in order to reduce soaking time before grinding and it also has gelatinous starch which gives added texture. Short or medium grained rice is used because you need over 80% amylopectin (s starch) to make soft and plump idlis. Long grain rice contains only 78% amylopectin and therefore is not suitable.
Idli rice is used to make idlis, fermented rice cakes which are normally served for breakfast in southern India and Ceylon.