Catechism of the Catholic Church §636 — excerpt
By the expression "He descended into hell", the Apostles' Creed confesses that Jesus did really die and through his death for us conquered death and the devil "who has the power of death" (Heb 2:14).
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.