Koshera system of rules and laws (the laws of kashrut) which for Jews govern the preparation and consumption of food.
Eating and drinking are for Jews religious acts where man takes from the bounty of God. Certain items are forbidden as food including blood, pork and fish without scales (e.g. shellfish). Other foods can be eaten separately but not together at the same meal. For example, milk cannot be eaten with meat, but each is a permissible food to be eaten separately within its own family of foods. The rules of kashrut are complex and in cases of doubt a Rabbi is consulted to make a decision according to the law.
Source: Rosten, The Joys of Yiddish.