From Albert Barnes notes on the bible:
To Baal - The word “Baal” in Hebrew means Lord, or Master. This was the name of an idol of the Phenicians and Canaanites, and was worshipped also by the Assyrians and Babylonians under the name of Bel; (compare the Book of Bel in the Apocrypha.) This god was represented under the image of a bull, or a calf; the one denoting the Sun, the other the Moon. The prevalent worship in the time of Elijah was that of this idol.
From Adam Clarke's Commentaries on the bible:
Who have not bowed the knee - Baal was the god of Jezebel; or, in other words, his worship was then the worship of the state; but there were several thousands of pious Israelites who had not acknowledged this idol, and did not partake in the idolatrous worship.