When someone is baptized in the Catholic Church as a baby, their parents and godparents make promises on their behalf to live a good and faithful Catholic life. When they get older, usually around 7 or 8 years old, they have the opportunity to receive the sacrament of Confirmation.
Confirmation is when the person decides for themselves that they want to continue to follow the Catholic Church teachings and be a full member of the Church. They go to classes to learn more about their faith and how to make good choices in life.
During the actual Confirmation ceremony, the person stands before the bishop (a special leader in the Church) and says "I do" to questions about their faith. The bishop then anoints their forehead with special oil, called chrism, and says a prayer asking for the Holy Spirit to be with them always.
Confirmation is like saying yes to being a good Catholic and having the Holy Spirit help you along the way.