In Jewish tradition, the leather boxes worn on the arm and head during weekday morning prayer are called tefillin.
Tefillin contain parchment scrolls inscribed with passages from the Torah, including the words of the Shema. One box is strapped to the arm near the heart, and another is placed on the head. Their use is based on biblical passages that instruct Jews to bind these words as a sign on the hand and between the eyes.
A bar mitzvah may learn to put on tefillin as part of preparation for religious adulthood. In traditional practice, tefillin are worn during weekday morning services, but not on Shabbat or most major Jewish festivals. Customs differ, especially regarding who wears them and how training is introduced.
Tefillin are not the same as a mezuzah, which is attached to a doorway, or a tallit, which is a prayer shawl. The boxes are sometimes called phylacteries in English, though many Jews prefer the Hebrew term tefillin.