On hearing this they were baptized in the name of the Lord Jesus. And when Paul had laid his hands upon them, the Holy Spirit came on them..(Acts 19:5-6)
Even though the apostles had lived with Jesus for three years, been witnesses of His miracles, and saw Him risen from the dead, it was not until they received the outpouring of the Holy Spirit on the Day of Pentecost that they were filled with courage to proclaim the Gospel to those around them. The Sacrament of Confirmation is the reception of that same Spirit sending us out into the community to tell others of the Risen Lord.
Confirmation is one of the three Sacraments of Initiation. It strengthens the Gifts of the Holy Spirit received at Baptism.