sweet red bean paste
Adzuki beans cooked down with sugar into a thick, sweet paste, either sieved smooth or left coarse with whole beans.
Sweet red bean paste is made from small red adzuki beans that are boiled until soft, thickened with sugar, and then either sieved smooth or only roughly mashed. It tastes gently sweet, slightly earthy and nutty, with a dense, somewhat floury texture. In Korea it fills pastries such as fish-shaped waffles and steamed buns and is spooned over shaved ice; the smooth kind works better as a filling, the coarse kind as a topping. It is sold in Asian grocery stores in cans, pouches or tubs, often labelled smooth or chunky, and by mail order. Dried adzuki beans for making it yourself are also stocked by health food and organic shops.
Substitute:
The best option is to make it yourself from dried adzuki beans with sugar and a pinch of salt, which takes about two hours and comes very close to the bought product. No ready-made ingredient replaces it properly: sweetened chestnut purée has a similar density and sweetness but tastes clearly of chestnut rather than bean.