Jewish kosher foods are those that have been prepared under the supervision of Orthodox rabbis who have studied the dietary requirements of the Hebrew scriptures and will guarantee that the rules have been followed.
When matzah (bread made without leaven) is prepared, for example, the rule is ancient and simple: flour and water must be mixed together and put in the oven within seventeen minutes. Any longer than that and the bread has a chance to rise, at which point it is no longer unleavened bread. Every place in the kitchen that might possibly contain yeast residue is examined. Ovens and all utensils are thoroughly cleaned to remove any possible trace of leaven. Timers keep careful watch on the clock. Every Jew in the world knows that on Passover her bread is really unleavened if it is stamped with the kosher seal.
From wine to pickles, every food has its traditional rules. Animals have to be slaughtered in a certain ritualistic way. Some foods cannot be mixed.
Only flesh of animals that have a "cloven foot and chew the cud" are ritually pure. (Cattle and deer—yes. Pigs—no.) Water animals must have both fins and scales. (Fish—okay. Lobster—forbidden.) Birds of prey are out, as are reptiles. Blood from any animal is not kosher, and meat must be drained and salted before cooking. Meat and milk (and foods derived from each) must never be mixed. In other words, cheese on a hamburger is not kosher, nor is any kind of meat on a pizza. Foods such as fruits and vegetables occupy a neutral ground and are considered pareve.