Yahweh compares the tightness of their neck muscles and the hardness of their foreheads to the hardness of iron and bronze. Here, to have a tight neck or a hard forehead is a metaphor that means the people are stubborn. AT: "it is as if your necks were iron and your heads were bronze" (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-metaphor]])