See also: Article on Garum ; Garum Recipe
Garum is a condiment or sauce widely used by the ancient Greeks and Romans.
Take some small fish, the intestines of some larger fish, maybe some oysters, salt them, maybe add some vinegar, pepper and/or other spices. Then set this in the sun for several days. At this point it is called liquimen.
As this appetizing mass of stuff would ferment and putrefy it oozed a liquid. This liquid is garum.
Used as a seasoning in cooking and also as a table condiment.
There are several modern versions of this: Pissalat from Nice and nuoc-mam in Vietnam are two of them.