Transubstantiation is the term used to explain the change that actually happens to the bread and wine offered at Mass into the Body and blood of Christ.
At the Last Supper, while holding bread and a cup of wine, Jesus told his disciples, “This is my Body” and “This is the cup of my blood.” Catholics repeat this ritual each time Mass is celebrated. What Catholics believe happens is that the substance of the bread and wine changes into the actual body and blood of Jesus when the priest recites the entire Eucharistic prayer. “Substance” here means what something is in itself despite how it’s perceived outwardly by the senses. Philosophers use the term “accidents” to describe what is perceptible to the senses. So while the accidents of bread and wine remain apparent to our senses, their essences are hidden to the senses but, nonetheless, are different in reality.
Each time Catholics celebrate Mass, transubstantiation occurs during the Eucharistic prayer when the priest repeats the words of Jesus and offers the sacrifice on behalf of the people present there. People then receive the body and blood of Jesus at Communion.
Catechism of the Catholic Church:
1376 The Council of Trent summarizes the Catholic faith by declaring: “Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.” 206