What is it's significance? What does it tell us about the God of Israel? The quote below is from Reading the Shema for all It's Worth by Jared Wilson
The original “creed,” full of truth and beauty.
“Hear, O Israel . . .” tells us theology isn’t just to be thought about but shouted about. Implicit in this proclamation is the assumption that those it is for will hear it. We proclaim God’s word with confidence knowing that it contains the Spiritual power to be heard by those with the ears to hear. So “hear this” isn’t just a figure of speech like “hear ye, hear ye” but a confession that true Israel will receive the speech in her spiritual senses. “Hear, O Israel” is a bit like “Come forth!” from the mouth of an empty tomb.
“The LORD our God, the LORD is one” tells us who God is, what God is, and how God is.
Firstly, who is this God? He is the LORD: he is YHWH. He has given us his name. He is not Baal or Belial or some indistinct Star-Warsian pantheistic life force. He is spirit but he is Personal — his name is YHWH, the great I AM. He tells us who he is.
Secondly, what is YHWH? Well, he’s God. Immortal, invisible, omnipotent, omnipresent, omniscient, holy, and sovereign. Gracious and merciful, just and jealous. Immutable, eternal, ineffable, transcendent. The God named YHWH is God.
Finally, how is God? “The LORD is one.” This God named YHWH is the God, the only God, the one true God. Sure, there’s plenty of so-called “gods,” but there is just one real God (1 Corinthians 8:5-6). So “the LORD is one” tells us how many Gods there are. But it also tells us how many Gods the three Persons the Scriptures identify as God amount to. “The LORD is one” tells us quantity but also unity. The three Persons proclaimed God in the Bible — Father, Son, and Holy Spirit — are one God.
There is more to the Shema, but this is a lot in itself.