Glatt kosher is a higher kosher standard which is used when inspecting large animals such as cattle after slaughter to determine whether or not their meat i up to the standards of the Jewish dietary restrictions. In order to be considered glatt, when an animal's lungs are examined, they must be smooth and free of defects. If the lungs have adhesions, punctures, or other defects, the meat is considered treif, “torn,” or forbidden.
There is some confusion as to what glatt kosher really means, which can cause some interesting labeling situations. Specifically, adult cattle and buffalo can be inspected to determine whether or not they are glatt. There are situations in which meat may not be glatt, but it could still be kosher. Smaller animals and fowl must always be glatt to be considered kosher. If chickens, ducks, calves, sheep, goats, deer, and so forth are treif, the meat is not kosher, and it cannot be eaten by Jews who adhere to kashrut, the Jewish dietary laws.
It is common to see butchers advertising glatt kosher meats to assure their Jewish customers that the meat is kosher. In the case of small animals, this is a bit misleading, as people may believe that the meat is extra kosher because it is described as glatt, when in fact if the meat wasn't glatt, it wouldn't be kosher at all. When large animal meat is advertised as being glatt kosher, the additional label carries more weight, distinguishing two different kinds of kosher meat.
While the rules surrounding glatt kosher meat might seem a bit arcane, there is a solid logic behind them. “Glatt” is Yiddish for “smooth,” referencing the even appearance of the lungs of a glatt kosher animal. Lungs which are smooth and free from adhesions are more likely to be healthy, suggesting that the host animal was also healthy, and not exposed to harmful substances which could have damaged its lungs.
Given the rising use of factory farming to produce meat, the idea of seeking out especially healthy animals has merit, and it suggests a level of attention which ordinary consumers may not exert. Textual support for the glatt kosher rules can be found in the Torah, a Jewish religious text, in which people are specifically forbidden to eat meat which is treif.