The Catechism in Questions & Answers

310. What is the significance of the Second Commandment in the Old Testament?

When God identified Himself to Moses in the burning bush, He gave His name. “I AM WHO I AM.” Here the name is not merely a mark of distinction. This name also describes the being of its bearer. Thereby God announces that He is changeless and eternal in His being. Individual human beings experience God in different ways, but God nevertheless remains changeless.
The being and majesty of God must not be infringed in any way. Out of reverence, devout Jews never utter the name “I AM WHO I AM” (Hebrew: Yahweh) aloud. Thereby they seek to avoid any—even unintentional—misuse of God’s name.

“And God said to Moses, ‘I AM WHO I AM.’ And He said, ‘Thus you shall say to the children of Israel, “I AM has sent me to you”’.”
Exodus 3: 14