The Mitzvah of Mezuzah in the simplest of words is to inscribe the words of the Shema onto your doorpost.
Therefore impress these My words upon your very heart: bind them as a sign on your hand and let them serve as a symbol on your forehead, and teach them to your children—reciting them when you stay at home and when you are away, when you lie down and when you get up; and inscribe them on the doorposts of your house and on your gates—
From these two prakim we gain the base for this mitzvah, "inscribe this on your doorposts". The text of Berachot 15b:23 gives a little more of an idea of what the mitzvah entails.
From this text, we learn that it is crucial to have all of the Shema in the mezuzah (and in Tefillin) and to not skip over any part of it. There is an obligation to have the entire Shema on our doorposts of our houses and gates. This metaphor can be applied to Judaism as a whole, where we should do our best to best Jews we can, without leaving out any important mitzvot or practices.
The Chizkuni explains that we do this mitzvah to remember that Hashem is One and is unique, both when you enter your house and when you leave it and brings up the connection between the only other time the Torah uses the word “מזוזות” and how it refers to the sides of the doorposts when talking about putting the blood on doorpost for Pesach. The only idea we get from his commentary on what the mitzvah looks like (rather than why we do it or connections that can be made to it) is that the Mezuzah should be affixed at shoulder level. This implies that the mezuzah had become a separate piece of something with the words of Shema inscribed upon it by the time that Chizkuni came around rather than being carved into wooden or stone doorposts.
מזזות ביתך. מזזת כְּתִיב, שֶׁאֵין צָרִיךְ אֶלָּא אַחַת:
Rashi teaches us that we only have one mezuzah per doorpost. He wants to insure that we know that only one mezuzah is needed for any doorpost one might have. This idea is nice because it is practical, and helps us do the mitzvah of Mezuzah without overcomplicating it. We can learn from this that God does not want us to struggle with mitzvot, and that we should not have to struggle with them.
In modern times this mitzvah provides a connection to the ancient text of the Shema in a beautiful way that we have elevated as time has gone on. Over time we have given people more access to having this mitzvah not be a burden and it is meaningful yet simple at the same time.
One difficulty of this tradition is that people in today’s society may have trouble taking a moment to appreciate God while living their everyday lives.
There are a lot of customs associated with mezuzahs that aren’t commandments, but are still widely practiced. The most popular custom is to touch the mezuzah every time you pass it and then kiss your fingertips as a sign of affection for the sacred prayers held inside. There’s a similar custom with Torah scrolls.
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“The principal behind the mezuzah is a beautiful thing,” says Shapero. “Whether or not you’re Jewish, the focus on family, on tradition and on teaching respect for others inside your home is very important. Jews are the only people who are commanded to do so, but others are welcome to join us in this tradition.”
This mitzvah is designed to be a simple way for us to connect with God multiple times a day, while not taking overly much time to do so. It is a simple action that can mean so much more than it seems at face value of taking notice of a mezuzah or kissing it as one walks through the doorway.
In the modern world and contemporary society it is possible to use this mitzvah as a simple raminder of God’s precence within our lives. We can put mezuzot on doorposts around our homes, and inother places of Jewish importance.
This mitzvah can teach us that mitzvot in general do not have to be particularly time-consuming or stressful, but can be small rituals that we do that are meaningful and elevate the spaces around them. The idea that we become holy by doing mitzvot relates to the idea of a mitzvah (or mitzvot elevating a space). Rabbi Elliot Dorff writes “We become holy as individuals if we abode by the law, but we also become holy as a people when we act in an exemplary manner…” (Rabbi Elliot Dorff, For the Love of God and People, "Motivations to Live by Jewish Law", pp.165-166).