Mild oil from soybeans, mostly sold refined as a light everyday cooking oil for stir-frying and cold dishes. In some dialects 豆油 means soy sauce instead.
In standard Mandarin 豆油 means soybean cooking oil. However, in Northeastern and some other regional dialects, 豆油 is used to mean soy sauce (standard 酱油) - context usually makes the meaning clear.