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.
English translation
"Answer us (aneinu), HaShem, answer us, on this day of our fast, for we are in great distress. Do not pay attention to our wickedness; do not hide Your face from us and do not ignore our supplication. Please be near to our outcry; please let Your kindness comfort us; before we call to You, answer us, as it is said: ‘And it will be that before they call, I will answer; while they yet speak, I will hear.’ For You, God, are the One who responds to His people Israel in time of distress, who redeems and rescues in every time of distress and woe. Blessed are You, God, who responds to His people Israel in time of distress."