What does Deuteronomy 19:14 forbid moving?

Answer: A neighbor's boundary marker

"You must not move your neighbor's boundary marker, which was set up by your ancestors to mark the inheritance" (KJV: "landmark").

“You must not move your neighbor’s boundary marker, which was set up by your ancestors to mark the inheritance you shall receive in the land that the LORD your God is giving you to possess.”— Deuteronomy 19:14 (BSB)