Catechism of the Catholic Church §1215 — excerpt
This sacrament is also called "the washing of regeneration and renewal by the Holy Spirit," for it signifies and actually brings about the birth of water and the Spirit without which no one "can enter the kingdom of God."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.