Hear O Israel, G-d is our Lord, G-d is one. Ch.6/4

This is the mitzvah of Kriat Shema; accepting the Kingship and yoke of Heaven upon oneself. The custom is to place one's hand over one's eyes when one recites it. One reason for this is practical. To ensure we are free of distractions, for this mitzvah (commandment) can only be fulfilled if one concentrates on the meaning of the words. However, there is a further reason that it can help to enhance our concentration and penetrate to the core of the mitzvah.

When we say that G-d is one, we are not saying this to exclude the possibility that He is two or three, for this is absurd and unnecessary. Rather we are affirming that G-d manifests Himself as a unity in this world. There is a synthesis to everything that has and will happen. At the end of time we will merit to see this, as the prophet Zechariah said, "G-d will be King over the world---on that day G-d will be one and His name will be one. However, until then we cannot see the full picture. We see righteous people suffering and wicked people prospering---where is the justice and where is the synthesis? And so we cover eyes, as if to say that we are putting aside our limited and distorted view of the world, and we reaffirm our belief in the oneness of G-d. With this sublime thought we have humbled ourselves before G-d and are ready to accept upon ourselves His Kingship with sincerity.