Tefillin are not amulets. They are reminders of God's laws.
Reprinted with permission from Louis Jacobs, The Jewish Religion: A Companion, published by Oxford University Press.
What They Are
Tefillin are the cube-shaped black leather boxes, containing four scriptural passages, attached to the head and arm and worn during the morning prayers. It is purely coincidental that the word tefillin so closely resembles the word for prayer, tefillah, since, although eventually the tefillin were only worn for the morning prayer, in Talmudic times they were worn all day and had no special association with prayer.
As Maimonides (Mishneh Torah, Laws of Tefillin, 4.25-6) puts it: "Great is the sanctity of tefillin, for as long as the tefillin are upon man's head and arm, he is humble and God-fearing and is not drawn after frivolity and idle talk, and does not have evil thoughts, but directs his heart to words of truth and righteousness. Therefore a man should try to have them on him all day ... Even though they should be worn all day it is the greater obligation to wear them during prayer." In point of fact, some few extremely pious individuals, even in post-Talmudic times, did wear tefillin all day and this seems to have been Maimonides' own practice. But the vast majority of Jews only wear tefillin during the morning prayer.
Etymology and History
The etymology of [the term] tefillin is uncertain, but possibly is connected either with a Hebrew root meaning "to attach" or with a root meaning "to distinguish." If this is correct, tefillin mean either "attachments" to the body or else the means whereby the Jew is distinguished from Gentiles. "Tefillin" is usually translated in English as "phylacteries." This is based on the New Testament Greek: "But all their works they do to be seen of men; they make broad their phylacteries" (Matthew 23:5). This passage, hostile to the Pharisees, uses the Greek word, from which the English is derived, meaning "things which guard"; in other words, the tefillin are a kind of amulet to offer protection against the demonic powers; whereas in all the Jewish sources the tefillin serve, like the tzitzit, as a reminder of God's laws.
In four Pentateuchal passages it is stated that certain words should be on the hand and between the eyes. Many commentators, including Rashbam [Samuel ben Meir, 11th century Bible and Talmud commentator from France], hold that the plain meaning of these passages is that the words of the Torah should be constantly in mind, as in the verses: "Set them as a seal upon thy heart, as a seal upon thine arm" (Song of Songs 8: 6) and "Let not kindness and truth forsake thee; bind them about thy neck, write them on the table of thy heart" (Proverbs 3: 3).