Catechism of the Catholic Church §2838 — excerpt
This petition is astonishing. If it consisted only of the first phrase, "and forgive us our trespasses," it might have been included, implicitly, in the first three petitions of the Lord's Prayer, since Christ's sacrifice is "that sins may be forgiven."
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.