The Hebrew word Tefillin is commonly translated as Phylacteries. Phylacteries is Greek for amulet. Tefillin are two small black leather boxes that contain parchment scrolls inscribed with Biblical passages with black leather straps attached to them. They are wrapped onto the arm and the head of adult men at morning weekday prayers. Wearing Tefillin is a positive commandment for Jewish males.
The word Tefillin is never mentioned in the Pentateuch. The commandment of Tefillin is due to Rabbinical interpretation of several sources in the Pentatech. The most explicit source is in Deuteronomy 6:8;
"You shall bind them as a sign upon your hand, and they should be for a reminder between your eyes."
Jewish males from the age of thirteen wear Tefillin. Thirteen is a Jewish males rite of passage into Jewish adulthood. This is the age at which he begins to be held accountable for his actions and conduct. Therefore he is thereon obliged to keep all laws pertaining to the Jewish people. One of these laws is Tefillin.
Tefillin are put on once a day, usually for morning prayers. One may not wear Tefillin at night. Tefillin are not worn on the Sabbath day, festivals or intermediary days on the Festivals of Passover and the Feast of Tabernacles. It should be noted that in some communities Tefillin are donned on intermediary days.
The procedure takes all of a few minutes but it is recommended to check one of the many clips on the internet with instructions on how to wear Tefillin. This way, one can copy that which he sees and be sure he is applying them correctly. Better still is if someone can show you in real life how to put them on.
The laws pertaining to creating Tefillin are complex. They are made by specially certified scribes with special ink and parchment. The houses of the parchment must be perfectly square. All letters on the parchment must be written perfectly. One incorrect letter can invalidate the entire pair of tefillin.
All these issues mean that one should not buy Tefillin from any source. Rather, one should take care to purchase only from a dependable vendor or website.
There are also different levels of quality of Tefillin. If a vendor tries to tell you to buy the more expensive kind because they last longer he is perfectly correct. Apart from this, there is also the aspect of doing commandments in the most beautiful way possible so if you can afford to buy more expensive ones do so and it is all the more praiseworthy.