That little black cube is called Tefillim. It consists of 2 sets of two small boxes and is one of the most important precepts (mitzvot) of the Torah. The Torah mentions it more than once, but more explicitly in Deuteronomy 6: 8 "You will bind them as a sign on your arm, and they will be a reminder between your eyes." Each of the two boxes contains four sections of the Torah written on parchment.
One of the straps is tied over the left arm (or right, if the person is left-handed) turning seven times and the other is placed over the head. According to Halacha, Jewish men from the age of thirteen must place them daily, with the exception of Shabbat and other Jewish holidays.
The word Tefilin passed to the Greek as Phylakterion (phylacteria) which is a diminutive of φυλακτήρ (phylakter), “Guardian”. Guardian of what?