The “source and summit” of the Church’s life, the Eucharist makes present the body and blood of Christ. Through the words of the priest at Mass, the bread and wine are transformed into Christ’s body and blood.
This is known as transubstantiation (i.e. a change of substance while the accidents or appearance remain the same). In the Eucharist, Jesus Christ is truly, substantially and really present and the bread and wine have ceased to exist. The fact of his being there is known as the Real Presence. At Holy Communion, one receives the body of Christ in its entirety.