What is "aneinu"

Aneinu, also transliterated as Aneynu or Anainu ( Hebrew for "answer us") is a Jewish prayer of atonement, asking God to forgive and protect his followers. It is most often recited by a cantor on public fast days including the Fast of Gedaliah, the Tenth of Tevet, the Fast of Esther, the 17th of Tammuz, and Tisha B'Av.

The prayer (translated below) is added into the weekday Shemoneh Esrei on the above fast days during the Afternoon service.