Confirmation is the completion of baptismal grace.
From Sacred Scripture: Acts 1:1-5
In the first book, O Theoph'ilus, I have dealt with all that Jesus began to do and teach, until the day when he was taken up, after he had given commandment through the Holy Spirit to the apostles whom he had chosen. To them he presented himself alive after his passion by many proofs, appearing to them during forty days, and speaking of the kingdom of God. And while staying with them he charged them not to depart from Jerusalem, but to wait for the promise of the Father, which he said, "you hear from me, for John baptized with water, but before many days you shall be baptized with the Holy Spirit."
Revised Standard Version, Second Catholic Edition
From the Rite of Confirmation
Those who have been baptized continue on the path of Christian initiation through the sacrament of confirmation. In this sacrament they receive the Holy Spirit, who was sent upon the apostles by the Lord on Pentecost.
This giving of the Holy Spirit conforms believers more perfectly to Christ and strengthens them so that they may bear witness 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.
Introduction, Dignity of Confirmation, Section 1-2, The Rites of the Catholic Church as Revised by the Second Vatican Ecumenical Council