Most people believe that this verse refers only about the unity of God (ECHAD God is one). However, that is only one of the 3 principles of the Pasuk Shema Israel. We also learn two more things:
One, ELOKENU, Hashem is our God: we assert our condition of chosen people, which implies a special relationship with God. Not a relationship of favoritism and more rights for us but mainly, chosen for greater responsibilities. We have 613 obligations toward God, while the rest of the world only 7.
The next lesson is kind of hiding in the original Hebrew and then it appears in English or vernacular: “HASHEM is our God…” HASHEM is… means, HASHEM exists.
So Chobot haLebabot says that when we say the first Pasuk of the Shema Israel, and every time we see or kiss the Mezuzah which contains the Shema Israel, we should think about these 3 principles of our Jewish faith:
1. God exists.
2. We are His chosen people.
3. God is ONE
Mezuzah joke: http://www.aish.com/j/j/51477127.html.