The Stone of Losses was a feature of the compound of the Temple in Jerusalem in ancient days. It was the "lost and found" spot where lost things were brought to the temple compound. Lost children and objects were brought to this large rock to be reunited with their parents and owners.
Professor Asher Kaufman maintains that this is the same as the large rock that is the center of the Dome of the Rock in Haram al-Sharif, i.e. the Temple Mount, however Judaism generally regards that as the Temple's "Foundation Stone".
"At the Stone of Losses" is a book of Jewish poetry.
References
The Stone of Losses Wikipedia(Text) CC BY-SA