Confirmation perfects Baptismal grace; it is the sacrament which gives the Holy Spirit in order to root us more deeply in the divine filiation, incorporate us more firmly into Christ, strengthen our bond with the Church, associate us more closely with her mission, and help us bear witness to the Christian faith in words accompanied by deeds.
1316 Catechism of the Catholic Church

 

The Sacrament of Confirmation confers special graces of the Holy Spirit upon the candidate being confirmed, in the same way as graces were granted to the Apostles on Pentecost. As with Baptism, it can only be performed once, and Confirmation increases and deepens all of the graces granted at Baptism.

As St. Thomas says:

Confirmation is to baptism what growth is to generation. Now it is clear that a man cannot advance to a perfect age unless he has first been born; in like manner, unless he has first been baptized he cannot receive the Sacrament of Confirmation (Summa Theologiæ III.72.6).