Confirmation, Baptism, and the Eucharist form the Sacraments of Initiation that are all intimately connected. In the Sacrament of Confirmation, the baptized person is "sealed with the gift of the Holy Spirit" and is strengthened for service to the Body of Christ. (Source: USCCB)
"Confirmation... imprints on the soul an indelible spiritual mark, the "character," which is the sign that Jesus Christ has marked a Christian with the seal of his Spirit by clothing him with power from on high so that he may be his witness."
- Catechism of the Catholic Church, paragraph 1304