"Hear O Israel" – is Moses' call to the entire Jewish people, and to every individual Jew, to hear these things.
This "hearing" is not merely with the ears, but rather it means listening, and beyond that – understanding and acceptance.
These words assume an additional meaning when shema is being recited, for then they serve as a declaration: Let the entire people of Israel hear the things which I, who utter them, proclaim and make known to all.
And for a person who recites shema by himself it is as if he were calling upon himself, saying: "Hear and listen, you who are a member of the people of Israel."
The next two words, "Hashem Elokaynu" – "the Lord our God" – are the essence of the "acceptance of the yoke of Heaven" by declaring that the Lord is our God, that we accept Him and are willing to take His rule upon us.
And in the last two words, "Hashem Echad" – "the Lord is One" – we state the main principles of the faith in God: His unity and His uniqueness.
In the words "The Lord is One" there are all the meanings of the term "One."
"One" does not only stand as against dualism (or trinity, or any other kind of plurality of gods).
It also includes God's one-and-only-ness (in the sense of "there is none else besides Him" – Deut. 4:35), and the sense that, compared with the "truth of His existence," no other reality counts.
–Rabbi Adin Steinsaltz
From an essay "The Portion of "Shema" by Rabbi Adin Steinsaltz