Confirmation takes 2 years or so of "Sunday school" and on the day the Confirmation is performed similar to a Baptism (Holy water on you but not sumergion).
It is some sort of Baptism, where you "confirm" or "reconfirm" your faith. Since that happens in your late teenage years but you can do it older (because Baptism is done when you're a baby or child).
The Catechism of the Catholic Church sees the account in the Acts of the Apostles 8:14–17 as a scriptural basis for Confirmation as a sacrament distinct from Baptism