Black soybeans are allowed to ferment and then salted. Sometimes a rice wine vinegar and flavourings such as garlic or ginger are added. The resultant sauce has a pungent, salty tang to it.
There are recipes for making your own, but since they all require fermented black beans for which you’d have to make a trip to an Asian market anyway, you might as well buy the sauce while you’re there. Asians do.
The sauce is very salty, so hold back on the salt in the dish until close to the very end when you can do a final taste and adjust seasoning.