Before Jesus was put to death, He promised His followers that He would send the Spirit to comfort and strengthen them. On the day of Pentecost, the Holy Spirit was poured out onto the Apostles. The sacrament of confirmation is our own Pentecost. When we are confirmed, the Holy Spirit is poured out onto us so that we receive the gifts of the Holy Spirit.
Confirmation is administered by anointing with the oil of Chrism and the laying on of hands. The Bishop places his hands on the heads of the candidates and then anoints their foreheads with holy oil, saying the words, "Be sealed with the Gift of the Holy Spirit." In Jesus' time, soldiers were marked with their leader's seal and slaves with their master's. By receiving the seal of the Holy Spirit, we show that we belong to God. Being anointed is a sign of ownership and belonging, but it also elevates us to a higher position. In the Old Testament, priests, prophets, and kings were all anointed with oil. When we receive the oil on our foreheads, we become part of the priesthood of all believers, witnesses to Christ and heirs to His royal throne.
Confirmation is the third and last of the three Sacraments of Inititation along with Baptism and the Eucharist.
With the sacrament of Confirmation, the baptized now become a true witness of Christ, and charged with a higher obligation to spread and defend the faith through word and deed. The seeds of the sacrament of Confirmation can be traced back to the Acts of the Apostles, specifically Acts 8:14-17, which states that "Now when the apostles in Jerusalem heard that Samaria had accepted the word of God, they sent them Peter and John, who went down and prayed for them, that they might receive the Holy Spirit, for it had not yet fallen upon any of them; they had only been baptized in the name of the Lord Jesus. Then they laid hands on them and they received the Holy Spirit."
--Faith for the Future: A New Illustrated Catechism