What are Selichot?

Selichot (Hebrew:סליחות) is a series of penitential prayers Piyyutim which are recited on a daily basis at early morning services during the month of Elul.

Selichot, meaning Prayers for Forgiveness, are ancient prayers mentioned in the Mishnah, as prayers for fast days.

The Mishnah describes the fast days and the order of prayer as featuring a series of exhortations that end with the words “He will answer us,” recalling the times in Jewish history when G-d answered the sons of Israel.

At the service, the people begin to examine their deeds of the past year, seeking forgiveness from G-d and promising to improve their behavior in the New Year.
Selichot are recited between midnight and dawn. Some prefer to recite it at night after the Arvit service.

The Selichot service emphasizes the recitation of The Thirteen Attributes: a list of G-d’s thirteen attributes of mercy that were revealed to Moses by G-d after the sin of the golden calf: HaShem, G-d, merciful, gracious, long-suffering, abundant in goodness, truth, keeping mercy unto the thousandth generation, forgiving iniquity, transgression, sin, who cleanses.

Credit:
my Jewish learning