This is a Favorite Stove cast iron skillet with sulphur pitting on the bottom. It was used in the early 20th century on a gas stove, when the natural gas used in that stove had a fairly high sulphur content. The sulphur reacted wth the iron and literally dissolved the metal, producing this pitting on the underside. This is permanent damage and it can't be restored. Nonetheless, this pan can still be used for cooking, and there's no reason not to continue using it.