Sometimes United Methodists use the word “confirmation” as reference to the class or preparatory time before the ritual of confirmation. This time or group experience should more accurately be called “confirmation preparation.” 

What is the person preparing for? 
By Water and the Spirit: A United Methodist Understanding of Baptism describes it this way:

When persons who were baptized as infants are ready to profess their Christian faith they participate in the service, which United Methodism now calls Confirmation. This occasion is not an entrance into Church membership, for this was accomplished through baptism. It is the first public affirmation of the grace of God in one’s baptism and the acknowledgment of one’s acceptance of that grace by faith. This moment includes all the elements of conversion-repentance of sin, surrender and death of self, trust in the saving grace of God, new life in Christ, and becoming an instrument of God’s purpose in the world. The profession of Christian faith, to be celebrated in the midst of the worshiping congregation, should include the voicing of baptismal vows as a witness to faith and the opportunity to give testimony to personal Christian experience.

Two things should be noted here:

One, confirmation is not becoming a member of the church –
baptism already celebrated that!

Two, confirmation is a person’s first public affirmation of the faith of the church as being his or her own faith.

In other words, the confirmand is taking responsibility for living as a member of the body of Christ and for fulfilling God’s purposes.

By Water and the Spirit illuminates this further:

An infant who is baptized cannot make a personal profession of faith as a part of the sacrament. Therefore, as the young person is nurtured and matures so as to be able to respond to God’s grace, conscious faith and intentional commitment are necessary. Such a person must come to claim the faith of the Church proclaimed in baptism as her or his own faith. Deliberate preparation for this event focuses on the young person’s self-understanding and appropriation of Christian doctrines, spiritual disciplines, and life of discipleship. It is a special time for experiencing divine grace and for consciously embracing one’s Christian vocation.

So, confirmation preparation aims at leading youth and others to embrace their vocation-the calling of God to live and witness to the reign of God announced in Jesus Christ.