When my daughters moved into a house while going to college, there was this metal case on the doorpost. It was mysteriously removed at some time and I then investigated as to what it was. Below is a description from a Judaism 101 website.
It's sad that some really do not know what idolatry really means.
On the doorposts of traditional Jewish homes , you will find a small case .This case is commonly known as a mezuzah (Heb.: doorpost), because it is placed upon the doorposts of the house. The mezuzah is not, as some suppose, a good-luck charm, nor does it have any connection with the lamb's blood placed on the doorposts in Egypt. Rather, it is a constant reminder of G-d's presence and G-d's mitzvot.
The mitzvah to place mezuzot on the doorposts of our houses is derived from Deut. 6:4-9, a passage commonly known as the Shema (Hear, from the first word of the passage). In that passage, G-d commands us to keep His words constantly in our minds and in our hearts, by (among other things) writing them on the doorposts of our house. The words of the Shema are written on a tiny scroll of parchment, along with the words of a companion passage, Deut. 11:13-21. On the back of the scroll, a name of G-d is written. The scroll is then rolled up and placed in the case, so that the first letter of the Name (the letter Shin) is visible (or, more commonly, the letter Shin is written on the outside of the case).
The scroll must be handwritten in a special style of writing and must be placed in the case to fulfill the mitzvah. According to traditional authorities, mechanically printed scrolls do not fulfill the mitzvah of the mezuzah, nor does an empty case.
The case and scroll are then nailed or affixed to the right side doorpost on an angle, with a small ceremony called Chanukkat Ha-Bayit (dedication of the house - yes, this is the same word as Chanukkah, the holiday celebrating the rededication of the Temple after the Maccabean revolt against Greece). A brief blessing is recited. See the text of the blessing at Affixing the Mezuzah.
Every time you pass through a door with a mezuzah on it, you touch the mezuzah and then kiss the fingers that touched it, expressing love and respect for G-d and his mitzvot and reminding yourself of the mitzvot contained within them.
It is proper to remove a mezuzah when you move, and in fact, it is usually recommended. If you leave it in place, the subsequent owner may treat it with disrespect, and this is a grave sin. I have seen many mezuzot in apartment complexes that have been painted over by subsequent owners, and it breaks my heart every time I see that sort of disrespect to an object of religious significance.
The Shema also commands us to bind the words to our hands and between our eyes. We do this by "laying tefillin," that is, by binding to our arms and foreheads leather pouches containing scrolls of Torah passages.
The word "tefillin" is usually translated "phylacteries," if you don't already know what tefillin are, and partly because it means "amulet," and suggests that tefillin are some kind of protective charm, which they clearly are not. On the contrary, the word "tefillin" is etymologically related to the word "tefilah" (prayer) and the root Pe-Lamed-Lamed (judgment).
Like the mezuzah, tefillin are meant to remind us of G-d's mitzvot. At weekday morning services, one case is tied to the arm, with the scrolls at the biceps and leather straps extending down the arm to the hand, then another case is tied to the head, with the case on the forehead and the straps hanging down over the shoulders. Appropriate blessings are recited during this process. The tefillin are removed at the conclusion of the morning services. See a general outline of this process and its blessings at Tallit and Tefillin.
Like the scrolls in a mezuzah, the scrolls in tefillin must be hand-written in a special style of writing.