Catechism of the Catholic Church §1224 — excerpt
Our Lord voluntarily submitted himself to the baptism of St. John, intended for sinners, in order to "fulfill all righteousness." Jesus' gesture is a manifestation of his self-emptying.
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.