XO Sauce is a spicy seafood sauce that can be used as an ingredient or a table condiment.
It is made from dried shrimp, dried scallop, garlic, chile, onion, dried fish, and oil. In restaurants, the sauce may include some brandy, either in the sauce or by virtue of brandy being in the dish it’s being added to.
Bottled versions are available, though expensive. Some (such as Lee Kum Kee Brand) have red chile pepper and ham in them. Bottled versions may also have added modified corn starch, colouring, etc.
Another well-known brand is Amoy.
XO Sauce seems to have first appeared in Cantonese cooking in Hong Kong in the 1980s, made in-house in restaurants.
XO Sauce is named after the grade of brandy, XO, which is seen as quite exclusive in Hong Kong. In Hong Kong, XO has come to mean “quality, luxury.”