What is Tefillin?

(4) HEAR, O ISRAEL: THE LORD IS OUR GOD, THE LORD IS ONE. (5) And thou shalt love the LORD thy God with all thy heart, and with all thy soul, and with all thy might. (6) And these words, which I command thee this day, shall be upon thy heart; (7) and thou shalt teach them diligently unto thy children, and shalt talk of them when thou sittest in thy house, and when thou walkest by the way, and when thou liest down, and when thou risest up. (8) And thou shalt bind them for a sign upon thy hand, and they shall be for frontlets between thine eyes. (9) And thou shalt write them upon the door-posts of thy house, and upon thy gates.

(16) Take care not to be lured away to serve other gods and bow to them. (17) For the LORD’s anger will flare up against you, and He will shut up the skies so that there will be no rain and the ground will not yield its produce; and you will soon perish from the good land that the LORD is assigning to you. (18) Therefore impress these My words upon your very heart: bind them as a sign on your hand and let them serve as a symbol on your forehead, (19) and teach them to your children—reciting them when you stay at home and when you are away, when you lie down and when you get up; (20) and inscribe them on the doorposts of your house and on your gates— (21) to the end that you and your children may endure, in the land that the LORD swore to your fathers to assign to them, as long as there is a heaven over the earth.

The phylactery of the hand is bound on the left arm over the biceps muscle which is the flesh that stands out on the upper arm that is between the shoulder blade and forearm, so that when the upper arm is held close to the ribs, the phylactery will be opposite the heart and thus the precept will be fulfilled, " And these words ... shall be upon your hear." (Deut 6:6)