Something that was beyond your ability to fully understand or comprehend. This is what Isaiah is trying to describe here. He saw the glory of the Lord, the magnificence of the Lord of Hosts.
In the year of the death of Uzziah the king, I saw the Lord sitting on a high and raised throne, and the hem of his robe was filling the temple.
He saw God on His throne saw the Lord sitting upon a throne, God has all authority, all power and deserves all honor and praise. This throne was massive, "His robe filled the temple" just imagine the most elaborate wedding gown and a train 20'. What Isaiah saw was so infinitely grander than the most elaborate dress or robe that man could ever produce.
Bless Yahweh, O my soul. O Yahweh my God, you are very great. You clothe yourself with splendor and majesty,
It declares His royalty so far beyond any royalty on earth.
The earth is Yahweh’s, with its fullness, the world and those who live in it, because he has founded it on the seas, and has established it on the rivers. Who may ascend the mountain of Yahweh? And who may stand in his holy place? He who is innocent of hands and pure of heart, who does not lift up his soul to falseness, and does not swear deceitfully. He will receive blessing from Yahweh, and justice from the God of his salvation. Such is the sort of those who seek him, those who seek your face, even Jacob. Selah
Seraphs were standing above him. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And the one called to the other and said, “Holy, holy, holy is Yahweh of hosts! The whole earth is full of his glory.”
Then Isaiah sees the seraphim, their call shook the foundations and this massive room was filled with smoke. This was the point at which Isaiah came to the full realization of his own uncleanness. It was because he saw God's holiness on such a grand scale that I think it shook the foundation of his soul. The statement "the whole earth is full of his glory." is more of a declaratio
And I said, “Woe to me! For I am destroyed! For I am a man of unclean lips, and I am living among a people of unclean lips, for my eyes have seen the king, Yahweh of hosts!”
This led him to a confession and also the realization of the condition of the people in his community.
Then one of the seraphs flew to me, and in his hand was a hot coal he had taken from the altar with tongs.
Then grace was given to Isaiah as one of the seraphim took a piece of coal from the alter and forgave his sins by touching it to his lips.
Then I heard the voice of the Lord saying, “Whom shall I send? And who will go for us?” And I said, “I am here! Send me!”
Upon receiving forgiveness for his uncleanness Isaiah eagerly answers the call to proclaim God's message to the people.
And he said, “Go and say to this people, ‘Keep on listening and do not comprehend! And keep on looking and do not understand!’ Make the heart of this people insensitive, and make its ears unresponsive, and shut its eyes so that it may not look with its eyes and listen with its ears and comprehend with its mind and turn back, and it may be healed for him.”
I tried to imagine my response to the seeming hopelessness in the message Isaiah had to deliver. Isaiah having just realized his own uncleanliness and the feeling of being forgiven but then knowing that the people you wish to proclaim a message to will not be able to hear or understand. I believe this is what causes his response in vs.11
Then I said, “Until when, Lord?” And he said, “Until the cities lie wasted without inhabitant, and houses without people, and the land is ruined and a waste, and Yahweh sends the people far away, and the abandonment is great in the midst of the land. And even if only a tenth part remain, again she will be destroyed like a terebinth or like an oak, which although felled, a tree stump remains in them. The seed of holiness will be her tree stump.”
"How long, O Lord", it's his plea filled question to God to find out how long will the people not be able to hear his message. God's answer was filled with destruction and exile for all but a few and "The holy seed".
God's glory is His holiness manifested.