For many Jews it begins with a set of penitential prayers called Selichot.
Selichot are prayers for pardon and mercy
Britannica defines selihoth, or selichot, as penitential prayers originally composed for Yom Kippur and fast days and later incorporated into the services preceding Rosh Hashanah and continuing through the Ten Days of Penitence.
That is the basic frame. Selichot are preparatory prayers of repentance.
They help begin the moral work before the holidays
Britannica's High Holy Days overview notes that penitential prayers are recited before the daily morning service in this season. Chabad adds a practical distinction: Sephardic communities begin much earlier in Elul, while Ashkenazic communities begin later.
So Selichot are not one fixed custom in every community, but they serve the same purpose: moral preparation.
The prayers keep repentance from becoming last-minute
This matters because Jewish repentance is not meant to start on Yom Kippur afternoon. Selichot extend the runway.
Why they still matter
Selichot still matter because serious repentance usually needs rehearsal, repetition, and language before the climactic fast day arrives.
The shortest accurate answer
Selichot are Jewish penitential prayers recited in the period before Rosh Hashanah and through the Days of Awe as preparation for repentance.