It's a kind of soy sauce-like sauce, which on the one hand enhances the color of the sushi, and on the other hand, diners don't need to dip into the soy sauce anymore, so the rice won't fall on the soy sauce dish. The sauce is made differently in each restaurant, and is usually flavored with bonito, kombu, sugar, and cooking wine, which is mostly used in high-class sushi restaurants. The other answers had salad oil in them, which was a real laugh...