That is a reference to the commission of Isaiah (Isaiah 6). When he suddenly found himself in the immediate presence of the exalted God (likely Jesus), he was undone and fully aware of the separation between himself and the LORD. The coal touching his lips was a sign of the purification Christ provides, after which Isaiah can say "Here I am, send me!"
So I said: "Woe is me, for I am undone! Because I am a man of unclean lips, And I dwell in the midst of a people of unclean lips; For my eyes have seen the King, The Lord of hosts." Then one of the seraphim flew to me, having in his hand a live coal which he had taken with the tongs from the altar. And he touched my mouth with it, and said: "Behold, this has touched your lips; Your iniquity is taken away, And your sin purged." Also I heard the voice of the Lord, saying: "Whom shall I send, And who will go for Us?" Then I said, "Here am I! Send me" [Isaiah 6:5-8]
