A mezuzah is a sacred parchment inscribed by hand with two portions of Torah. It is stored in a protective case and then hung.
The two portions of Torah written on the parchment are Shema (Deuteronomy 6:4-9) and Vehaya (Deuteronomy 11:13-21). Both of these Torah portions include the verse, "And you shall inscribe these words upon the doorposts of your house and upon your gates."
In accordance with this commandment, the mezuzah in this photo was hung on Jaffa gate, one entrance to the Old City of Jerusalem.