John Gill 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)

For he built up again the high places which Hezekiah his
father had destroyed
The temples and altars upon them, see (2 Kings 18:4),

and he reared up altars for Baal ;
in the high places he rebuilt:

and made a grove, as did Ahab king of Israel :
which was either an idol itself, or a shade of trees where idols were placed; or rather Asherah, rendered "a grove", is the same with Astarte, the goddess of the Zidonians, the figure of which he made and worshipped; for groves were not so soon and easily planted, raised, and made; so the same in (1 Kings 16:33)

and worshipped all the host of heaven, and served them :
the sun, moon, and stars, particularly the planets Saturn, Jupiter, Mars, Mercury, and Venus.