Iodised salt (also spelled "iodized salt") is table salt mixed with tiny amounts of iodine and is used to prevent disease of the thyroid gland.
People only need a very small amount of iodine to prevent disease. Food crops usually get iodine from the soil, but some places around the world have low amounts of iodine. Iodised salt is a very cheap and good way of getting the necessary iodine.
Ordinary table (cooking) salt is a chemical compound called "sodium chloride" (its chemical formula is NaCl). "Sodium iodide" (NaI) is added to the NaCl to make iodised salt.