Salt is a mineral, and as such, pure salt is always kosher. Some brands of salt have a kosher symbol on the package, and that way you know that a reliable kosher certification agency is checking to make sure that nothing else gets mixed in to the salt and that it’s 100% kosher.
In truth, the name “kosher salt” is misleading. A better term would be “koshering salt.” Blood is not kosher. G‑d commands us in the Torah:1 “You shall not eat any blood, whether that of fowl or of beast, in any of your dwellings.”
After a kosher animal is properly slaughtered, all blood must be removed. This is normally accomplished by salting the meat, as salt draws out blood. Table salt is too thin and will dissolve into the meat without drawing out the blood, and salt that is too coarse will roll off. The salt that is “just right” for koshering meat is called “kosher salt.”
Many chefs and recipes call exclusively for kosher salt, for two main reasons:
1. Table salt is iodized, meaning extra iodine is added during the processing, which affects the flavor. Kosher salt has a more pure salty flavor.
2. The larger crystals make kosher salt easier to use. You can pick it up and sprinkled it with your fingers, giving you a better feel for how much you’ve used, whereas table salt usually needs to be poured and can be harder to track. It is also easier to distribute evenly, which doesn’t matter much in a soup or stew, but does matter when it comes to roasted vegetables, for example.