Holy Eucharist refers to the three aspects of Christ’s body and blood — as sacrifice during the Consecration of the Mass, as Holy Communion, and as Blessed Sacrament. These three aspects form the core of Catholic belief on the Holy Eucharist.
Only wheat bread and grape wine can be used. The moment the priest or bishop says the words of consecration — the words of Christ at the Last Supper, “This is My body” and “This is My blood,” (Matthew 26:26–29) — Catholics believe that the bread and wine become the body and blood, soul and divinity of Christ.
On the natural level, whatever we eat becomes part of us (that is, until there’s too much of us, and then we must go on a diet). On the supernatural plane, when Catholics eat the body and blood of Christ, they’re supposed to become more like Christ in his obedience to the Father, humility, and love for neighbors.