With the last two words of the Shema, "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-onlyness (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.
The word "One" also contains the idea that the One God is all-inclusive.
The Rishonim (ancient commentators) say that when uttering these words during the recitation of Shema one should intend to mean that God is One in the seven heavens and on earth.
Therefore the Halacha says that one should prolong the uttering of the word "echad", "One," so that he can think of all the meanings of God's unity.
In the name of the Holy ARI it is said that when saying this verse, one should identify with its message completely, namely that the praying person should think and feel that he is willing to give his very life and suffer anything rather than swerve from this declaration that "The Lord our God, the Lord is One."
–Rabbi Adin Steinsaltz
From an essay, "The Portion of Shema" (Deuteronomy 6:4-8), by Rabbi Adin Steinsaltz