The meaning of the word confirm given in the dictionary is to make stronger or more persistent and this is what people are doing with their religious faith when they are confirmed. People are often (though not always) baptised when they are babies, so confirmation is their chance to renew for themselves the promises made for them by their parents and godparents when they were christened.
People are not usually confirmed before the age of about 13 but many are adults before they are confirmed.
The confirmation itself is a fairly simple ceremony, normally incorporated into the ordinary church service but taken by the Bishop, who has higher authority in the church than the parish vicar or regular minister.