The short answer: It is the Rite of Christian Initiation of Adults and it is the way people become Catholic today.
A longer and more complete answer: RCIA is an official series of stages and celebration of “Rites” which move adults, and older children, from questions about God and the Catholic Church to full participation in the Church through the Sacraments of Initiation (Baptism, Confirmation, and the Eucharist).
Becoming a Catholic is a spiritual journey that believers have traveled for more than 2,000 years. This journey means spending time in prayer, thought and reflection. It means becoming part of a community of women and men who are also on a journey of faith. God offers the gift of faith to all people. The essence of the Catholic faith is the belief in Jesus Christ, who became man to reveal truth about a loving and forgiving God. RCIA is a journey of faith and a process of conversion to Jesus for:
those who have not been baptized in any church, but would like to investigate or pursue the possibility of becoming Catholic.
those who have been baptized in a Christian (but not Catholic) church, and would like to investigate or pursue the possibility of becoming Catholic.
those who were baptized in the Catholic Church, but never completed their initiation by celebrating the Sacraments of Confirmation and/or Eucharist.
those who are married or dating a Catholic and would like to learn more about their loved one’s faith.