The De Profundis takes its name from the first two words of the psalm in Latin. It is a penitential Psalm that is sung as part of Vespers (Evening Prayer) and in commemoration of the dead. It is also a good Psalm to express our sorrow as we prepare for the Sacrament of Confession.

Every time you recite the De Profundis, you can receive a partial indulgence (the remission of a portion of punishment for sin). In the modern numbering of the Psalms, the De Profundis is listed as Psalm 130, though you will often see it listed as Psalm 129, according to the traditional numbering.

Out of the depths I cry to You, O Lord; Lord, hear my voice. Let Your ears be attentive to my voice in supplication. If You, O Lord, mark iniquities, Lord, who can stand? But with You is forgiveness, that You may be revered. I trust in the Lord; my soul trusts in His word. My soul waits for the Lord more than sentinels wait for the dawn. More than sentinels wait for the dawn, let Israel wait for the Lord, for with the Lord is kindness and with Him is plenteous redemption, and He will redeem Israel from all their iniquities.

Hail Mary for Souls in Purgatory
St. Faustina's Prayer