Concise Bible Dictionary:

2. asherah, asherath. The word “grove” naturally suggests a row of trees, but that this cannot be the meaning is evident from groves being set up “under every green tree” (1 Kings 14:2323For they also built them high places, and images, and groves, on every high hill, and under every green tree. (1 Kings 14:23); 2 Kings 17:1010And they set them up images and groves in every high hill, and under every green tree: (2 Kings 17:10)). Manasseh set a graven image of the grove that he had made in the temple, which Josiah removed, burnt, and ground to powder (2 Kings 21:77And he set a graven image of the grove that he had made in the house, of which the Lord said to David, and to Solomon his son, In this house, and in Jerusalem, which I have chosen out of all tribes of Israel, will I put my name for ever: (2 Kings 21:7); 2 Kings 23:66And he brought out the grove from the house of the Lord, without Jerusalem, unto the brook Kidron, and burned it at the brook Kidron, and stamped it small to powder, and cast the powder thereof upon the graves of the children of the people. (2 Kings 23:6)). This was doubtless made of metal, but the groves were of wood, as we learn from their being cut down, and burnt (Judg. 6:25-2625And it came to pass the same night, that the Lord said unto him, Take thy father's young bullock, even the second bullock of seven years old, and throw down the altar of Baal that thy father hath, and cut down the grove that is by it: 26And build an altar unto the Lord thy God upon the top of this rock, in the ordered place, and take the second bullock, and offer a burnt sacrifice with the wood of the grove which thou shalt cut down. (Judges 6:25‑26); 2 Kings 23:14-1514And he brake in pieces the images, and cut down the groves, and filled their places with the bones of men. 15Moreover the altar that was at Beth-el, and the high place which Jeroboam the son of Nebat, who made Israel to sin, had made, both that altar and the high place he brake down, and burned the high place, and stamped it small to powder, and burned the grove. (2 Kings 23:14‑15)). One passage speaks of groves being planted (Deut. 16:2121Thou shalt not plant thee a grove of any trees near unto the altar of the Lord thy God, which thou shalt make thee. (Deuteronomy 16:21)); another, of their being made, and another, of their being built (1 Kings 14:15, 2315For the Lord shall smite Israel, as a reed is shaken in the water, and he shall root up Israel out of this good land, which he gave to their fathers, and shall scatter them beyond the river, because they have made their groves, provoking the Lord to anger. (1 Kings 14:15)
23For they also built them high places, and images, and groves, on every high hill, and under every green tree. (1 Kings 14:23)
). They are constantly associated with idols and images, and Judges 3:77And the children of Israel did evil in the sight of the Lord, and forgat the Lord their God, and served Baalim and the groves. (Judges 3:7) speaks of their being served along with Baalim.
On the whole it seems most probable that they were wooden symbols of a goddess, in the form of images or pillars, or mere stems of trees inserted in the earth. In 2 Kings 23:77And he brake down the houses of the sodomites, that were by the house of the Lord, where the women wove hangings for the grove. (2 Kings 23:7) we read that women wove “hangings” for the groves, but these were literally “houses” or “tents,” which implies that they enclosed the groves, probably for impure purposes, for immorality was almost constantly associated with idolatry. Kalisch and others suppose that the name Asherah has reference to the Syrian goddess Astarte, and it is so translated by the LXX in 2 Chronicles 15:1616And also concerning Maachah the mother of Asa the king, he removed her from being queen, because she had made an idol in a grove: and Asa cut down her idol, and stamped it, and burnt it at the brook Kidron. (2 Chronicles 15:16). Fürst refers it to the Phoenician nature-god. The many references to the idols, images, and groves show how far Israel had departed from the living God and fallen into idolatry.