God is Holy. That means that He is set apart from all else and is in a category of perfection all by Himself. It is also means that God is morally pure and righteous, completely devoid of anything wrong, evil, or bad.

If the holy God were just a frail old man in the corner of a desert somewhere, it wouldn’t impact us much. But the holy God is your Creator and the your final Judge. Therefore God’s holiness requires a response of us.

After God demonstrated His majestic Holiness to the nation of Israel at Mount Sinai and proclaimed His moral Holiness to them in His Law, the people responded instinctively in this way:

Now when all the people saw the thunder and the flashes of lightning and the sound of the trumpet and the mountain smoking, the people were afraid and trembled, and they stood far off and said to Moses,
You speak to us, and we will listen; but do not let God speak to us, lest we die.”
Moses said to the people,
Do not fear, for God has come to test you, that the fear of him may be before you, that you may not sin.

Exodus 20:18-20

An almost identical response to God’s Holiness characterised Isaiah’s experience.

And I said:
Woe is me! For I am lost; for 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 burning coal that he had taken with tongs from the altar. And he touched my mouth and said:
Behold, this has touched your lips; your guilt is taken away, and your sin atoned for.

Isaiah 6:5-7

What do you do with a holy God? You shudder at the thought of such holiness, you recognise your own unholiness, and you gratefully accept His forgiveness of your sins, purposing to put an end to your own sinful deeds.