A Covenant

"The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament." CIC, can. 1055 § 1; cf. GS 48 § 1.

For a brief explanation of the Sacrament of Marriage, the Catechism of the Catholic Church is a good place to start.
https://www.vatican.va/archive/ENG0015/__P50.HTM