Albert Barnes Commentary Exodus 3:14

"And God said unto Moses, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you." — Exodus 3:14 (ASV)

I am that I am - That is, “I am what I am.” The words express absolute, and therefore unchanging and eternal Being. The name, which Moses was thus commissioned to use, was at once new and old; old in its connection with previous revelations; new in its full interpretation, and in its bearing upon the covenant of which Moses was the destined mediator.