CCC 1289 “In the West, Confirmation suggests both the ratification of Baptism, thus completing Christian initiation, and the strengthening of baptismal grace.” Basically, that is what Confirmation is, in a nutshell. However, I’m sure you would like some history and background, so here you are:
"Very early, the better to 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.’ This rite of anointing has continued ever since, in both East and West. For this reason the Eastern Churches call this sacrament Chrismation, anointing with chrism, or myron which means ‘chrism.’ In the West, Confirmation suggests both the ratification of Baptism, thus completing Christian initiation, and the strengthening of baptismal grace - both fruits of the Holy Spirit.
Two traditions: East and West
In the first centuries Confirmation generally comprised one single celebration with Baptism, forming with it a “double sacrament,” according to the expression of St. Cyprian. Among other reasons, the multiplication of infant baptisms all through the year, the increase of rural parishes, and the growth of dioceses often prevented the bishop from being present at all baptismal celebrations. In the West the desire to reserve the completion of Baptism to the bishop caused the temporal separation of the two sacraments. the East has kept them united, so that Confirmation is conferred by the priest who baptizes. But he can do so only with the ‘myron’ consecrated by a bishop." (CCC 1289-1291)