It means “Done and dusted”, i.e. “there’s nothing more to do—move on.”
It’s taken from the last two verses of Psalms 128, which describes the cherished vision of a future where the people of Israel are blessed, and all is good in the world—a kind of utopia, where we needn’t strive any more:
ה יברכך יהוה מציון: וראה בטוב ירושלים–כל ימי חייך.
ו וראה-בנים לבניך: שלום על-ישראל.
5 The Lord shall bless thee out of Zion: and thou shalt see the good of Jerusalem all the days of thy life.
6 Yea, thou shalt see thy children’s children, and peace upon Israel.