The account of Hezekiah's reform in the eighteenth chapter of Second Kings is brief and startlingly specific. He removed the high places, the local altars where Judahites had lawfully sacrificed for centuries. He broke the standing stones and cut down the wooden cult pole. And he broke in pieces the bronze serpent that Moses had made, because until then the Israelites had been burning incense to it, and he called it Nehushtan, a piece of bronze.
That last act is unlike anything else in the Hebrew Bible. The bronze serpent belongs to a story in Numbers in which the Lord instructs Moses to make it, and looking at it heals those bitten by snakes in the wilderness. It is therefore an object made at divine command, by Moses, and preserved in the sanctuary, and Hezekiah destroys it because of what people had come to do with it. The reasoning is not that the object was false but that veneration had attached to the thing rather than to what it pointed at, and the name he gives it, reducing it to its material, makes the point.
What the reform meant in practice is the harder question. Concentrating sacrifice in one place removes the local shrine from the village and puts the priesthood, the calendar and the revenue under central control, so the measure is at once theological and political, and modern historians debate how much of it happened under Hezekiah and how much the account projects backwards from the later reform of Josiah. Excavated evidence is suggestive rather than decisive: an altar at Tel Beersheba was dismantled and its stones reused, and the sanctuary at Tel Arad went out of use, but dating either event to a particular king's decree remains contested.