Tefillin are one of the more distinctive parts of Jewish ritual when non-Jews (Gentiles) think about Judaism. Tefillin are a pair of black leather boxes containing Hebrew parchment scrolls. A set includes two—one for the head and one for the arm. Each consists of three main components: the scrolls, the box and the strap. The tefillin consist of two black leather boxes and straps to hold them on. One is worn on the biceps, and its strap, which is tied with a special knot, is wound by the wearer seven times around the forearm and hand—on the left arm for right-handed people and on the right for those who are left-handed. The second box is worn on the forehead at the hairline, with its straps going around the back of the head, connected at the top of the neck with a special knot, and hanging in front on each side.
In the Torah Jewish men are commanded to bind tefillin onto their head and upper arm every weekday, in fulfillment of the verse (Deut. 6:8), “You shall bind them as a sign upon your hand, and they shall be for a reminder between your eyes.” Most men wear tefillin in Orthodox and Conservative congregations, as do some women in Conservative congregations. The use of tefillin is less prominent in Reform and Reconstructionist congregations by both men and women.
What’s in the boxes you ask? The Torah mentions the mitzvah of tefillin four separate times. Each of these texts is inscribed on parchment and placed into the tefillin. These verses cover the fundamentals of the Jewish faith. These texts are:
1–2. Kadesh (Exodus 13:1–10) and Vehayah ki yeviacha (Exodus 13:11–16): These describe the duty of the Jewish people to always remember the redemption from Egyptian bondage, and the obligation of every Jew to educate his children about this and about G‑d’s commandments.
3. Shema (Deut. 6:4–9): Pronounces the unity of the one G‑d, and commands Jews to love and fear Him.
4. Vehayah (Deut. 11:13–21): Focuses on G‑d’s assurance to Jews of reward that will follow their observance of the Torah’s mitzvahs.
In order to be kosher according to Jewish law, tefillin must meet thousands of requirements. If one part is out of place, the whole thing won’t work. The scrolls inside the tefillin are inscribed in black ink with a quill (or reed) pen by a specially trained scribe, known as a sofer. The parchment is handmade and must be from a kosher animal. There are 1594 letters in each of the tefillin boxes. If one letter is extra, missing, or even incorrectly written, the tefillin are invalid. The boxes and straps are also made of leather from a kosher animal. The head-tefillin is made of four separate compartments, each one containing a scroll with one of the four Torah selections. The hand-tefillin has just one chamber, with all selections written on a single scroll. You’ll notice that the head tefillin has the Hebrew letter Shin (ש) on both sides, one with three branches, and the other with four.
The hand-tefillin are strapped onto the left arm (or on the right arm, for a lefty), with the box resting on the bicep, facing the heart. The rest of the strap is then wound around the arm seven times, extending down to the long finger. There are many customs regarding how the coils are positioned on the arm and fingers. The head-tefillin are placed on the head like a crown, with the box resting just above the hairline in the center of the forehead.
Sources : “Essential Judaism” Robinson, George. 2016. myjewishlearning.com. chabad.org