Hebrew Letters'Shin' , 'Daled' , 'Yod' which pronounced as Shaday
his is the abbreviation of SHOMER DLATOT ISRAEL or Guardian of the Doors of Israel - one of the names of G-d.
This name is usually appears on Jewish Mezuzah - the doors guardians. The mezuzah is also a symbol of God's watchful care over the home. The name of God,Sha-dai, which appears on the reverse side of the parchment, is an acronym for the Hebrew words which mean Guardian of the doorways of Israel. The placing of a mezuzah on the doors of a home or office protects the inhabitants -- whether they are inside or outside.