The Psalter refers to the Book of Psalms, the collection of 150 sacred songs and poems found in the Old Testament. The Catechism calls the Psalter the "masterwork of prayer in the Old Testament," capturing the full range of human emotion and experience before God—from joyous praise to agonizing lament (CCC 2585).
The Prayer of Christ and the Church
The Psalms were the prayers of Jesus Christ Himself, who prayed them throughout His life and quoted them from the Cross. Because of this, the Psalter remains the foundational prayer book of the Catholic Church. It forms the core of the Liturgy of the Hours (the Divine Office), through which the Church continuously sanctifies time and unceasingly offers praise to the Father in union with Christ (CCC 2586–2588).
May the Lord bless you and keep you.