“Mezuzah” literally means a doorpost. We are directed by the Torah to affix a sacred parchment on our doorposts, upon which are written the first two paragraphs of the Shma. This is an expression of our fundamental beliefs, including Hashem’s unity, sovereignty, and our wholehearted love and devotion for Him. The scroll, together with the respectful protective case around it, is universally called a “Mezuzah”. The mezuzah reminds us of Hashem regularly. There is also a widespread custom to kiss the mezuzah when passing it, expressing our love for Hashem.
A Symbol of Faith and Identity
The prominent display of the mezuzah on our doorways is an eternal symbol of Jewish identity on every continent and in every age. Gouged out hollows in the doorways of Vilna, Cordova, Baghdad, and other cities around the world wordlessly recount the poignant story of the stops along our people’s arduous journey. Even today, as we drive around our cities, we unconsciously keep a lookout for mezuzot on doorposts as tell-tale signs of Jewish presence.
But our mezuzah is more than merely a display of Jewish identity. It broadcasts our identification with the fundamental beliefs and principles of our people and its historic destiny. Ramban passionately asserts in his classic Torah commentary:
For he who purchases a mezuzah for a small coin, affixes it to his doorway, and contemplates its message has acknowledged the Creation, Divine Providence, and Prophecy. Indeed, he has proclaimed his belief in all aspects of the Torah. 
- Shimshon Raphael Hirsch, in his monumental work Horeb, classifies the mitzvah of mezuzah as a “testimony,” a symbolic observance representing truths which form the basis of Jewish life. It shares this designation with Shabbat and Yom Tov, which continually rejuvenate our connection to our historic mission and our destiny:
The Biblical passages “Shema Yisrael” and “V’haya im shamo’a” should be written on the entrances of every house, thereby hallowing the house (and indeed every place specially set aside for human activities) as an abode where Hashem is ever present and where service of Hashem is fulfilled, thus testifying that all one’s life, all that one endures, is accomplished through Hashem. 
The significance of the Jewish doorway as both a portal to our inner life and a broadcaster of our identity to the outside emerges right from the dawn of our history. Indeed, at the first Pesach seder, way down in Egypt land, Hashem commanded that we daub the paschal lamb’s blood on our doorposts and lintel to mark the inviolate sanctuary of the Jewish home. In our times, as well, the inscription of the Divine Name ש-ד-י on the back of the mezuzah parchment indicates that Hashem’s presence follows us in all our wanderings. As Talmud Yerushalmi teaches:
The Holy One Blessed Be He has attached His great name to Israel. This can be compared to a king who possessed a small key to his palace. He said, “If I leave it as it is, it will be lost. I shall make for it a chain, so that if it is lost, its chain will identify it.” In the same way, Hashem said, “If I leave Israel on their own, they will be swallowed up among the nations. Rather, I will attach my Great Name to them, and they shall survive!”
Perching silently and unobtrusively on our doorposts, the mezuzah is far from a passive adornment or a talisman. Rather, we are challenged to engage with its message with our minds — and even with our emotions. Our encounter with mezuzah is meant to ignite a spark of love deep within our hearts. Rambam accordingly chose to incorporate the laws of mezuzah in The Book of Love, the second book of his monumental Mishneh Torah. As he explains in his introduction: “I will include within it [the book] all the Mitzvot… which were commanded to us so that we will love Hashem and constantly remember Him.” 
This “constant remembrance” is a defining feature of mezuzah. Once one affixes a mezuzah to his doorway, the mitzvah, its effect, and its reward keep pumping while he goes about his daily life and even while he sleeps. This quality, together with the fact that mezuzah is incumbent on all Jews, indicates to us that Hashem considers this mitzvah to be essential and indeed indispensable to our lives: Women, as well as men, are obligated in the mitzvah as it is not time-bound. Even children must be trained in its observance. As Rambam teaches:
A person must show great care in [the observance of the mitzvah of] mezuzah because it is an obligation which is constantly incumbent upon everyone.
The classic work Chovos HaLevavos illustrates this concept with a fascinating observation. It notes that in the Creator’s wisdom he has provided us with resources in proportion to their necessity. Because a constant supply of oxygen is an absolute prerequisite for survival, He has surrounded us with a vast atmosphere and arranged for its replenishment. Similarly, life-sustaining water is abundantly distributed around the globe and renewed through the rain cycle.  In the same way, the constancy and universality of mezuzah indicate that its observance is crucial for our spiritual nourishment. Sefer HaChinuch asserts that with time the mezuzah’s very presence will inevitably influence our character and inner awareness. 
For this reason, the Sages prescribe that we should place the mezuzah at the outermost handbreadth of the doorpost so that we will encounter it immediately upon entering.  As Rambam explains: “…whenever a person enters or leaves [the house], he will encounter the unity of the name of the Holy One, Blessed Be He, and remember his love for Him.”
 Ramban, Commentary to the Torah, Shemos 13:16.
 Horeb, Intro. to Section II, p. 187.
 Yerushalmi, Pei’ah 2:6 cited in Sha’arei HaMezuzah p. 182.
 Rambam, Introduction to Mishneh Torah.
 Rambam, Hilchos Sefer Torah 5:10.
 Rambam, Mishneh Torah, Hilchos Sefer Torah 6:13.
 Chovos HaLevavos, Sha’ar HaBechinah.
 Sefer HaChinuch, Positive Mitzvah 16.
 Menachos 33b.