Catechism of the Catholic Church §1625 — excerpt
The parties to a marriage covenant are a baptized man and woman, free to contract marriage, who freely express their consent; "to be free" means: - not being under constraint; - not impeded by any natural or ecclesiastical law.
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.