Yakiniku Sauce is a range of sauces used to accompany a Japanese or Korean Yakiniku-style meal. Most Westerners, though, tend of think of a garlic-flavoured sauce when they think of Yakiniku Sauce.
They can be made or bought: there are many commercial brands available. Many are soy based, though some such as Akadare made by Kikkoman are miso-based. Japanese Yakiniku Sauces ones tend to use more soy sauce than do Korean versions of the sauces. Some are even made with fruit.
Yakiniku Sauce is used both as a marinade, a grilling sauce and a dip. Most are just thick enough to coat a piece of meat, though some particularly fruity ones will be thicker.
A Japanese-style Yakiniku sauce:
4 tablespoons hoisin sauce
4 tablespoons soy sauce
4 tablespoons mirin
1 tablespoon sesame oil
2 tablespoons lime juice
5 tablespoons chopped fresh coriander leaves
1 tablespoon fresh ginger, peeled and minced
2 tablespoons honey
Mix all ingredients except for the honey. Simmer until thickened. Strain, then add honey.