""You shall bind them as a sign on your hand, and they shall be a reminder between your eyes" (Deuteronomy 6:8).
You have a brain. It's in one world. Your heart is in another. And your hands usually end up wrapped up in something completely different from both of them. Three different machines.
Then you put on Tefillin. Almost first thing in the day, you connect your head, your heart, and your hands with those leather "wires," so that they all work in harmony, with the same intention, as preparation for facing the world.
Tefillin are a pair of black leather boxes containing parchment scrolls. One box is for the head, the other for the arm. Each has three basic components: the scrolls, the box, and the straps. The scrolls are inserted into the boxes, which are painted black.
They are placed on the head, in front of the heart, once a day, preferably during the morning prayer, while reciting the "Shema Yisrael". Men from the age of 13 do this every day, except Shabbat and Yom Tov.
This is what you should think about while putting on Tefillin: The Holy One, blessed be He, commanded us to write on the parchment containing the Tefillin the four biblical passages that mention His Oneness and the Exodus from Egypt, so that we may remember the miracles and wonders He performed for us. These indicate His Oneness and demonstrate that He possesses power and dominion over those above and below, to do with them as He wills. And He has commanded us to place the Tefillin on the arm near the heart, and on the head over the brain, so that we may submit our soul, which resides in the brain, as well as the desires and thoughts of our heart, to His service. Thus, by putting on Tefillin, you will keep the Creator in mind and restrain your desires.