The Catholic Church teaches that marriage is a lifelong union between a man and a woman. When two baptized Christians enter into marriage freely, understanding its sacredness, permanency and indissolubility, they become a symbol of a sacramental marriage. What does that mean?
According to the Catechism of the Catholic Church, sacraments are “actions of the Holy Spirit at work in his Body, the Church. They are ‘the masterworks of God’ in the new and everlasting covenant.” (no. 1116) Learn more about the concept of a sacrament.
Regarding the sacrament of marriage, the catechism says: "the sacrament of matrimony signifies the union of Christ and the Church. It gives spouses the grace to love each other with the love with which Christ has loved his Church ..." The grace of this sacrament "perfects the human love of the spouses, strengthens their indissoluble unity and sanctifies them on the way to eternal life." (no. 1661)