Kicking off his visit to the U.S., Pope Francis was scheduled to spend Wednesday meeting with President Barack Obama and leading a prayer with U.S. bishops at Saint Matthew’s Cathedral in Washington, D.C. Later in the week, the pope was expected to preside over a multi-religious service at the 9/11 Memorial and Museum in New York, and in Philadelphia, he was expected to celebrate a mass at the Basilica of Saints Peter and Paul. With so many religious services in the days ahead, here’s a look at some of the central beliefs, rituals and prayers of Roman Catholicism.

What Is Holy Communion? Also known as the Eucharist and Lord’s supper, Holy Communion serves as a reenactment of Jesus’s last supper with his disciples before his arrest and crucifixion. At the last supper, wine and bread were served with Jesus saying, “This is my body,” regarding the bread and, “This is my blood,” regarding the wine.

In church today, people taking part in Holy Communion eat consecrated bread and drink wine or grape juice. Catholics are encouraged to regularly receive Communion, and those receiving it should not be “conscious of grave sin and normally should have fasted for one hour,” according to the United States Conference of Catholic Bishops. Catholics believe celebrating Holy Communion shows a person’s “oneness of faith, life and worship.”

What Are The Key Prayers? For Catholics, prayer is considered to be a response to God who has already spoken or is still revealing himself. The most common prayers are “Our Father,” “Hail Mary,” “The Rosary” and “Apostles’ Creed.”

Prayers can also directed toward events and individuals such as prayers for troops and prayers for after an election.

When Do You Stand And Kneel During Mass? Catholics stand at different points during mass, including when the Gospel is read, to show reverence and respect. Throughout mass there are moments when people will kneel. Kneeling shows a person’s humility before God. These practices vary among churches and countries.