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).