A fine white alkaline powder. Chinese cooks rub a little into sliced beef so it stays soft in a stir-fry, and add it to doughs and batters to make them rise; too much tastes soapy.
苏打 is a loanword from English 'soda'. 小苏打 ('little soda') is sodium bicarbonate (baking soda); 大苏打 ('big soda') is sodium thiosulfate, used in photography. Both are old-school chemistry names.