(DAI-yahn ha-E-met) n. The Righteous Judge, referring to the LORD as the Sovereign over the lives and fortunes of all people. Baruch Dayan HaEmet is an blessing of submission said at a time of bereavement for the Jew.
The Talmud (Tractate Berachot 59b) states that you should say a blessing for sad news, ascribing to the LORD the glory due His Name even in personal loss.
בָּרוּךְ אַתָּה יְהוָה אֱלהֵנוּ מֶלֶךְ הָעוֹלָם דַיָּן הָאֱמֶת
Barukh attah Adonai Eloheinu melekh ha-olam, dayan ha-emet.
"Blessed art Thou, LORD our God, Master of the Universe, the Judge of the truth."