Why are there two types of the letter "ש" protruding from the Tefillin?
The three headed ש: In memory of the three fathers; Abraham, Isaac and Jacob.
The four headed ש: In memory of the four mothers; Sarah, Rivka, Rachel and Leah.
The three legged Shin is on the right and the four legged one is on the left.
Gematria: (assigns a numerical value to each letter in the Hebrew alphabet thus allowing compilations of these numbers.)
The letter ש has the numerical value of 300. Thus 2 x 300 = 600. When on reads the Hebrew word which contains two letters "Shin" שש you get the number 6.
The combined numbers of heads on the two "Shins" protruding from the Tefillin are seven.
Thus, we receive the following equation (300 x 2) + 6 + 7 = 613 which are the number of commandments that are incumbent on the Jewish people.
When we wear our Tefillin we are proclaiming our desire to keep and uphold these 613 mitzvot.
The Tefillin are a sign of the reincarnation
The procedure in which we insert the individual scrolls into the body of the Tefillin and the manner in which they are sewn together have deep connotations to the reincarnation.
The bodies of the scrolls are equated to the human body.
The parchment is equated to human skin.
The tendons used to sew together the Tefillin are equated to human tendons.
The straps inserted into the Tefillin are equated to clothing.
Thus, it is suggested that all those who fear G-d and wear Tefillin will be included in the reincarnation.
The Tefillin are a sign of G-d's wedding with the Jewish people
In the holy book "Song of Songs" the Jewish people are equated to a bride who is married to G-d. We therefore wrap the Tefillin straps seven times around our arms in commemoration of the seven blessings recited during the Jewish wedding ceremony. In, addition, the winding of the straps around the fingers commemorates the wedding ring given by the husband to the bride.
Laying of the Tefillin on the weaker hand.
We are commanded to lay the Tefillin on the weaker hand. Thus a right handed person lays his Tefillin on the left hand and a left handed person lays his Tefillin on the right hand. The reasoning behind this is to show that we cannot overcome our enemies without the grace of assistance of G-d.
Why are the skins and parts of animal used to manufacture Tefillin?
The animal symbolizes the animalistic, materialistic world that we live in.
The Bible and the mitzvot were given to the Jews in order to spiritually lift us from this materialistic world in to a spiritual world. We take a piece of an animal and insert into it spirituality. During the entire procedure of manufacturing the Tefillin we recite that "All the work being carried out today in manufacturing the Tefillin is for the holiness of the Tefillin.
In the same manner, our everyday lives can be transformed into a more meaningful spiritual life.