Albert Barnes Commentary 2 Kings 23:3

"And the king stood by the pillar, and made a covenant before Jehovah, to walk after Jehovah, and to keep his commandments, and his testimonies, and his statutes, with all [his] heart, and all [his] soul, to confirm the words of this covenant that were written in this book: and all the people stood to the covenant." — 2 Kings 23:3 (ASV)

By a pillar - Rather, “upon the pillar” (see 2 Kings 11:14, note).

Made a covenant - “The covenant.” Josiah renewed the old covenant made between God and His people at Horeb (Deuteronomy 5:2), so far as such a renewal was possible by the mere act of an individual. He bound himself by a solemn promise to the faithful performance of the entire Law.

With all their heart - “Their” rather than “his,” because the king was considered to be pledging the whole nation to obedience with himself. He and they “stood to it,” that is, they “accepted it and came into the covenant.”