The Sacrament of Confirmation confers special graces of the Holy Spirit upon the person being confirmed, just as such graces were granted to the Apostles on Pentecost. Like Baptism, therefore, it can only be performed once, and Confirmation increases and deepens all of the graces granted at Baptism.
The Catechism of the Catholic Church lists five effects of Confirmation:
It roots us more deeply in the divine affiliation [as sons of God]
It unites us more firmly to Christ;
It increases the gifts of the Holy Spirit in us;
It renders our bond with the Church more perfect;
It gives us a special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross.
Because Confirmation perfects our baptism, we are obliged to receive it "in due time." Any Catholic who did not receive Confirmation at baptism or as part of his religious education during grade school or high school should contact a priest and arrange to receive the Sacrament of Confirmation.