豉 is salted fermented beans and 油 is oil (also used in 香油sesame oil), so 豉油 is salted fermented bean oil.
About This Dish
Soy sauce is brewed from fermented soybean paste.
History
Soy sauce originated in China over two thousand years ago as a way to stretch the use of salt (a prized commodity in ancient China, heavily taxed and smuggled).