In his typically brilliant style, Rav S.R. Hirsch explains the meaning of a Mezuza as mentioned in this weeks Parshas Ekev, Devarim 6:9
The mezuzah is not an amulet; in and of itself, it does not protect the
house. Only insofar as they shape their lives in accordance with the mezuzah’s
content can the people within the house expect help and protection
from God, the “All-Sovereign and All-Sufficing,” in all the vicissitudes
of domestic life. With this intent it is our custom to adorn the
outside of the mezuzah with the Name shin-daled-yud.