This literally means "peaceful sabbath" in Hebrew.
This is the standard greeting between Jews on Shabbat, from sundown on Friday until sundown on Saturday, but is often used as early as Thursday.
Shalom: peace/completeness or wholeness and general greeting for all time.
Shabbat: the seventh day of the week for the Jewish calendar. A day off (literally a day to be set apart from all other days-for introspection, spiritual growth, and renewal) - beginning on Friday at sundown and ending on Saturday evening when three stars appear in the sky and can be viewed together.