In mainstream Rabbinic Judaism, a mezuzah is affixed to the doorpost of Jewish homes to fulfill the mitzvah (Biblical commandment) to “write the words of God on the gates and doorposts of your house” (Deuteronomy 6:9).
The word mezuzah literally means doorpost, but it has come to mean the parchment scroll placed on the doorpost that is inscribed with the verses from Deuteronomy that start with, “Hear O Israel, the Lord our God, the Lord is One.
The Hebrew letter Shin stands for the word Shaddai, a name for God.