The Sacrament of Confirmation is one of three Sacraments of Initiation in the Catholic Church. The Confirmed member of the Church is gifted with the special outpouring of the Holy Spirit, like the apostles on the day of Pentecost.
From this fact, Confirmation brings an increase and deepening of baptismal grace:
“Recall then that you have received the spiritual seal, the spirit of wisdom and understanding, the spirit of right judgment and courage, the spirit of knowledge and reverence, the spirit of holy fear in God's presence. Guard what you have received. God the Father has marked you with his sign; Christ the Lord has confirmed you and has placed his pledge, the Spirit, in your hearts.” (Catechism of the Catholic Church, 1303)