What makes kosher salt "Kosher"?

Kosher salt usually has no additives, and it has big crystals with large surface areas. This size and shape allows it to absorb more moisture than other forms of salt

Nearly all salt is Kosher already. “Kosher salt,” also called “Koshering salt,” is called such because it’s very useful for making meats Kosher—i.e. for extracting the blood from the meat.

This is according to KitchenSavvy and Wikipedia.

It doesn’t have iodine.

Bob’s right.