Because three Hebrews, Shadrach, Meshach, and Abednego, refused to obey King Nebuchadnezzar's order to fall down and worship the golden image he had set up, they were thrown into a blazing furnace (Daniel III). They emerged miraculously untouched.
Nebuchadnezzar, henceforth convinced of the strength of the Hebrew God, commanded respect for Him and for those who worshipped Him.
The miniature shows the crowned Nebuchadnezzar witnessing the miracle while the man in charge of stoking the fire shields his face. There are four, not three, Hebrews, and, in fact, the Book of Daniel relates the surprise of the King of Babylon when, instead of the three young men he had thrown into the fire, four appear.
"And the form of the fourth is like the Son of God." By his interpretation of this passage, the translator of the Vulgate gave medieval thought claim to a prefiguration of the Messiah.
The Limbourgs imaginatively allowed the smoke of the furnace to escape into the center column of the page from the preestablished frame.
Below the miniature is text from the Canticle of Three Hebrews in the Fiery Furnace, as it appears in the Book of Daniel.

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