GOD is holy.
The Bible asserts that GOD is holy. In fact, statements like these are repeated throughout the Scriptures:
44 For I am the Lord your God. You shall therefore sanctify yourselves, and you shall be holy; for I, the Lord your God, am holy. Neither shall you defile your souls with any creeping thing that creeps on the earth. 45 For I am the Lord who brings you up out of the land of Egypt, to be your God. You shall therefore be holy, for I, the Lord, am holy.
(Leviticus 11:44-45 OSB)
The Greek word that is translated "holy" in English is ἁγίου (hagios). The word means "sacred, set apart."
GOD is entirely unlike anything in nature. He is different. He is entirely other.
One thing that makes GOD holy is that He is 100% completely pure. There is no sin, evil or immorality in Him. This makes GOD different than all of human creation, for we all sin and fall short of GOD's glory.
Holiness also carries with it the idea of reverence. GOD is so much greater than we are in every way — His power, glory, righteousness, justness, love, beauty and truth. For this reason, among others, it is necessary for us as His creation to revere GOD and hold Him in high esteem.
In fact, GOD's holiness is so great that His character will not allow Him to be in the immediate presence of sin.
13 [GOD's] eye is too pure to see evil, and You are not able to look upon affliction.
(Habakkuk 1:13 OSB)
GOD's holiness is the only characteristic which is repeated sequentially for emphasis.
1 In the year King Uzziah died, I saw the Lord sitting on a throne, high and lifted up. The house was full of His glory. 2 Around Him stood seraphim; each one had six wings: with two he covered his face, with two he covered his feet, and with two he flew. 3 And one cried to another and said, “Holy, holy, holy is the Lord of hosts; the whole earth is full of His glory.”
(Isaiah 6:1-3 OSB)