Mitzvah Kotel Images
The Shema, שמע, is the most essential prayer in Judaism, and a declaration of faith in one God. A Jew is obligated to say Shema in the morning and at night. When in the most dire moments of despair and desperation, Jews have cried out to The Almighty for help.
The idea that "G-d is One" means not only that there is one G-d, but that G-d and the whole of creation are One. That there is nothing apart from G-d. Everything in the universe is totally dependent on G-d at every moment and G-d continuously keeps the universe in existence.