In order to produce pickled tofu, cubes of dried tofu are allowed to fully air-dry under hay and slowly ferment from aerial bacteria and fungal spores. Commercially available pickled tofu is made by using dry firm tofu that has been inoculated with the fungal spores of Actinomucor elegans, Mucor racemosus, or Rhizopus spp..
The dry fermented tofu is then soaked in brine, typically enhanced with Chinese rice wine, vinegar, chili peppers or sesame oil, or a paste made of rice and soybeans. In the case of red pickled tofu (traditional Chinese: 紅豆腐乳/南乳; pinyin: hóng dòufu rǔ/nán rǔ), red yeast rice (cultivated with Monascus purpureus) is added for color. Pickled tofu is generally sold in small glass jars.
Pickled tofu has a special mouthfeel similar to certain dairy products due to the breakdown of its proteins which takes place during the air drying and fermentation. Since it does not have a strong odor by itself, pickled tofu takes on the smells and taste of its soaking liquid. The texture and taste of pickled tofu resembles a firm, smooth paste not unlike creamy blue cheese. (Indeed, this kind of tofu is sometimes called "Chinese cheese" in English). When refrigerated, it can be kept for several years, during which time its flavor is believed to improve.
Pickled tofu is commonly added in small amounts, together with a little bit of its soaking liquid, to flavor stir-fried or braised vegetable dishes (particularly leafy green vegetables like water spinach). Often, it is eaten directly as a condiment with rice or congee.
( Above info from Wikipedia )