The verse "Yahweh spoke by his servants the prophets, saying," in 2 Kings 21:10 is a standard formula used in the Hebrew Bible to introduce a prophetic message. Let's break it down:

Yahweh: This is the personal name of God in the Hebrew Bible, often translated as "LORD" in English. It emphasizes that the message originates directly from the divine.

spoke: This indicates that God communicated directly to the prophets. The method of communication could vary (dreams, visions, direct voice, etc.), but the core idea is divine communication.

by his servants the prophets: This highlights the role of the prophets as intermediaries. They are chosen and dedicated individuals, serving God and acting as His mouthpieces to deliver His message to the people. Calling them "servants" emphasizes their humility and submission to God's will. The word "prophets" identifies them as individuals specifically tasked with conveying divine messages.

saying: This word is the standard way to introduce the direct words of God as spoken through the prophet. What follows this word is the message that God wants to communicate.

In the context of 2 Kings 21:10, the verse serves to introduce a condemnation of King Manasseh's wicked actions. It establishes that the criticisms and warnings about to be delivered are not merely the opinions of individuals but are divinely inspired judgments from God Himself.

Here's what it means in a nutshell:

Authority: The message is not from human opinion, but from God Himself.
Legitimacy: The prophets were legitimate messengers of God, chosen to deliver His word.
Significance: What follows is of utmost importance, as it represents God's judgment and warning.

Essentially, this phrase is a signal to the reader that what comes next is a message of divine authority and importance, delivered through God's appointed messengers, the prophets. This establishes the gravity and seriousness of the prophetic warnings that will follow in the verses after 2 Kings 21:10.