Albert Barnes Commentary 1 Kings 15:13

"And also Maacah his mother he removed from being queen, because she had made an abominable image for an Asherah; and Asa cut down her image, and burnt it at the brook Kidron." — 1 Kings 15:13 (ASV)

Asa degraded Maachah from the rank and state of queen-mother.

The word translated “idol” both here and in the parallel passage does not occur elsewhere in Scripture. It is derived from a root meaning “fear” or “trembling,” and may perhaps best be understood as “a fright” or “a horror.” Such a name would seem to apply best to a grotesque and hideous image like the Phthah of the Egyptians. She made it to serve in place of the ordinary “grove”—the asherah, or idolatrous emblem of Astarte (Exodus 34:13). Asa cut it down, for like the usual asherah, Maachah’s “horror” was fixed in the ground.

And burnt it at the brook Kidron—Similarly, when Josiah removed Manasseh’s “grove”—the asherah—from the house of the Lord, he brought it out to the brook Kidron and burned it there. The purpose was probably to prevent the pollution of the holy city by even the ashes from the burning.