Albert Barnes Commentary 2 Kings 21:3

"For he built again the high places which Hezekiah his father had destroyed; and he reared up altars for Baal, and made an Asherah, as did Ahab king of Israel, and worshipped all the host of heaven, and served them." — 2 Kings 21:3 (ASV)

The first step in re-establishing idolatry seems to have been the restoration of the high places where Yahweh was supposedly worshipped (2 Kings 18:22), but with idolatrous rites (1 Kings 14:23). The next step was to reintroduce the favorite idolatry of Israel, Baal-worship, which had formerly flourished in Judea under Athaliah (2 Kings 11:18) and Ahaz (2 Chronicles 28:2). After this, Manasseh seems to have especially adopted Sabaism, which had been previously unknown in Judea (compare 2 Kings 17:16 and note).

Worshipped all the host of heaven: Sabaism, or pure star-worship without images and astrological superstitions, included reverence for the sun, the moon, the chief stars, and the twelve signs of the Zodiac (2 Kings 23:5 note). The primary form of worship involved altars on which incense was burned (Jeremiah 19:13). These altars were placed either on the ground (2 Kings 21:5) or on the house-tops (2 Kings 23:12; Zephaniah 1:5).

The sun was worshipped with the face toward the east (Ezekiel 8:16), and chariots and horses were dedicated to him (2 Kings 23:11). The star-worship of the Jews more closely resembles an Arabian cult than an Assyrian or Chaldean one. It took hold at a time when Assyria and Babylonia had little communication with Judea—that is, during the reign of Manasseh. It likely crept in from the same source as the Molech worship, with which it is linked here and in 2 Chronicles 33:3–6.