“Tefillin” is actually plural for tefillah, prayer, but the concept is really neither singular nor plural. The tefillin, which are also called “phylacteries” (a Greek word deriving from “to guard”), are two small black leather boxes with parchment scrolls inside containing verses from the Torah.
One box is made for placing on the forehead (“shel rosh,” for the head), and the other is placed on the arm (“shel yad,” for the hand). They have leather straps which are wrapped or bound to affix them to the body, following the commandment that the words shall be bound as a sign upon the arm and between the eyes (Deut. 11:18; see page 155 et seq. of Siddur Lev Shalem).
Tefillin are used every morning except Shabbat and festivals.
Shel rosh tefillin have the letter shin (ש) on both the right and left sides, but the shin on the left has four branches instead of three. It also has four compartments (while shel yad has only one), into which four parchments are placed, with four Biblical passages: Exodus 13:1-10, Exodus 13:11-16, Deut. 6:4-9, and Deut. 11:13-21.
So why the four branches? Some rabbis have said that the total branches thus is seven, which is a powerful number. Others have said that the original tablets Moses carried down would have had four-branched shins, while the written Torah would have had three-branched shins. Some say the two letters represent the three patriarchs and four matriarchs. Still others say that it is meant to channel more energy to the head. There are many interpretations. Of course, the first time may merely have been a typo (or a scribe-o).
You can learn more about tefillin at the World Wide Wrap event on February 4, 2018, at Rodef Shalom, a collaboration between J-JEP and the Beth Shalom Men’s Club. There is so much more to learn.