For those who were baptised as infants, it is an opportunity for them to renew the vows that their parents and Godparents made on their behalf as the Baptismal vows are repeated by them again as part of the Confirmation ceremony. The Sacrament of Confirmation makes a Catholic a witness of Jesus in the world with the help and guidance of the Holy Spirit.
Taken from The Faith Of The Catholic Church a CTS Publication
156. What is Confirmation?
Confirmation is the Sacrament that perfects the grace of Baptism. It gives the Holy Spirit so as to root us more deeply in our relationship with God as His sons and daughters, to incorporate us more solidly in Christ, to strengthen our link with the Church by associating us more closely in her mission, and to help us to witness to the Christian faith in word and deed.