A particular gift of the Holy Spirit. The first grace of this sacrament is one of strengthening, peace and courage to overcome the difficulties that go with the condition of serious illness or the frailty of old age. This grace is a gift of the Holy Spirit, who renews trust and faith in God and strengthens against the temptations of the evil one, the temptation to discouragement and anguish in the face of death. This assistance from the Lord by the power of his Spirit is meant to lead the sick person to healing of the soul, but also of the body if such is God's will. Furthermore, "if he has committed sins, he will be forgiven."
Source: Catechism of the Catholic Church (Vatican.va)