We have a precept to read the Shema prayer twice a day. One during daytime to connect to the level of Zeir Anpin and one time at night against Malchut. The Shema prayer creates the unification of Day and Night, Light and vessel and for that we need to read it twice a day.
To have God’s protection on our homes we have the precept to fix Mezuzah on the posts of our doors so when we go out and come in we have protection.
Psalms 121 has six words with the root ‘שמר’ that means ‘protect’. It also means that the protection comes from the six sefirot of Zeir Anpin.
“יְהוָה יִשְׁמָר צֵאתְךָ וּבוֹאֶךָ מֵעַתָּה וְעַד עוֹלָם”
“YHVH will guard your going out and your coming in From this time forth and forever.”
God is our protector and always by the entrance of our homes. The reason for the Mezuzah that has both aspects of male and female is so we won’t forget God’s protection. Like the Tzitzit that men wear and see to remind them about God’s precepts and protection, so is the Mezuzah for all of us to see and remember who is our protector. The Mezuzah is a connection and reminder of our faith.
The Mezuzah holds in it a parchment with the prayer of the Shema Israel scribed by hand. Only the first three paragraphs the Shema are on the parchment. They correspond to יהו YHV that are the first three letters of the YHVH name.
When we look at the Mezuzah or touch it we connect ourselves, which is the aspect of Malchut to Zeir Anpin.
Below is a picture of a Mezuzah parchment, normally rolled and inserted into a glass / clear tube. On the back of the parchment we have the name שדי that is God’s name related to the Sefira of Yessod. It is also the initials of שומר דלתות ישראל, ‘protects the doors of Israel’