What does Isaiah 6:5 mean?

God has given Isaiah a glimpse of Himself sitting on a throne as strange, angelic beings call out to one another about the Lord's holiness (Isaiah 6:1–3).These beings have voices that shake the very foundations of the building (Isaiah 6:4). Standing in the presence of God's glory, Isaiah is suddenly overwhelmed with his unworthiness to be in such a place or see such things.

Isaiah cries out in despair. This is not only because common understanding among the Israelites was that those who see God in His full glory must die (Exodus 33:20). It's that given a glimpse of the true holiness of God instantly burns into Isaiah's inner being the depth of his sin. Seeing the Lord of hosts makes him aware that he deserves death because of his sin.

Specifically, Isaiah describes himself as a man of "unclean lips" surrounded by people of "unclean lips". The reason he is calling out his lips specifically is it is with the lips, our words, that we reveal who we are in our hearts. Jesus said, "For out of the abundance of the heart the mouth speaks" (Matthew 12:34). Hearing the pure praise of the Lord from the lips of the seraphim, Isaiah knew in an instant his own unworthiness. And he is not unique, the people of Israel are also unworthy.