God said in the Bible: ”I am who am!” What does that expression mean? ”I am who am!” 

What does this expression mean?

God expressed Himself like so in the Bible. But what I want to know is what does it mean? What is its intrinsic meaning?
God said :

I am that I am. [Exodus 3:14, KJV.]
I am that which I am. [Exodus 3:14, Young's Literal.]

'I am' expresses a personal existence, an individual being.
'That I am' or 'that which I am' expresses a form of being, a mode of existence.
Elsewhere, God reveals that he is eternal, he is the Creator of all things :

Hast thou not known? hast thou not heard, that the everlasting God, the LORD, the Creator of the ends of the earth, fainteth not, neither is weary? there is no searching of his understanding. [Isaiah 40:28, KJV.]

Jesus Christ, the Son of God, states 'God is Spirit' (John 4:24, KJV).
The mode of God's existence is eternal spirit.
As such, he is known by faith, not by human sight or by scientific knowledge.