Very early, to better signify the gift of the Holy Spirit, an anointing with perfumed oil (chrism) was added to the laying on of hands. This anointing highlights the name “Christian,” which means “anointed” and derives from that of Christ himself whom God anointed with the Holy Spirit. Innocent III wrote: “The anointing of the forehead with chrism signifies the laying on of hands, which is otherwise called confirmation, since through it the Holy Spirit is given for growth and strength.”
This giving of the Holy Spirit conforms believers more perfectly to Christ for the building up of his body in faith and love. They are so marked with the character or seal of the Lord that the sacrament of confirmation cannot be repeated.
By the sacrament of Confirmation, the baptized are more perfectly bound to the Church and are enriched with a special strength of the Holy Spirit. Hence they are, as true witnesses of Christ, more strictly obliged to spread and defend the faith by word and deed. – CCC 1285