This verse is part of the prophecy given to Huldah, a prophetess, during the reign of King Josiah in Judah. It's a direct response to Josiah's sending a delegation to inquire of Yahweh after the Book of the Law was rediscovered in the Temple. Let's break it down:

"But to the king of Judah, who sent you to inquire of Yahweh..." This establishes the context. The prophetess Huldah is being given a specific message for King Josiah. It acknowledges that it was his initiative to seek God's word through an inquiry. Josiah was taking responsibility and showing humility by seeking guidance.

"...you shall tell him this, ‘Yahweh, the God of Israel says:..." This emphasizes the divine origin of the message. Huldah isn't speaking on her own authority, but as a messenger of Yahweh, the covenant God of Israel. This is a formulaic introduction meant to give weight and authority to the prophecy.

"“About the words which you have heard..." This is a specific reference to the words in the Book of the Law that was recently found and read before Josiah. The message that follows will directly address the content of that law and its implications for Judah.
In essence, this verse sets up the delivery of a specific, divinely ordained message to King Josiah, addressing his concerns about the rediscovered law. It signals that the prophecy that follows will be directly relevant to Josiah's understanding of God's will for Judah.