Every transgression is idolatry and an attack on the divine Oneness and every commandment is a connection to this Oneness.
Concerning the meanings of echad ("One") and yachid ("Only"), whereas the concept of echad allows for the existence of an additional being, yachid denies any possibility of duality, a second and similar divinity.
When reciting the Shema, we say that "God is echad" rather than "God is yachid," even though the meaning of yachid is apparently more exalted than that of echad.
We say "God is echad" because echad expresses God's existence within the worlds (the four directions, the earth, the seven heavens).
In reciting Shema, we emphasize the inclusiveness and oneness of His all-encompassing being.
Yachid, on the other hand, expresses the singularity and exclusivity of God's essence and being before it was drawn into the worlds.
We believe that God is unique: not only is there no other divinity like Him but there is not even any existence outside of Him.
"He is All Alone" means that He alone exists, now no differently than before the world was created.
–Rabbi Adin Steinsaltz
From Learning from the Tanya, Chapter 20 by Rabbi Adin Steinsaltz