As I said at the very beginning of our considerations, in these little morceaux, of the Sacrament of the Eucharist, the central notion has to be this: our Eucharist is Christ Jesus Himself.
It is at Mass that mere wheat bread and grape wine is changed by Christ into Himself. The change is real, even though the outward appearance of bread and wine remains the same. Sometimes people make the mistake of thinking that the Eucharistic food only “reminds us of” Jesus, or “symbolizes” Him, or represents Him in some lesser way. This is not true. We know that Christ’s words at the Last Supper, recorded in the Bible, are direct and true ones: “This is my body, which will be given for you; this is . . . my blood, which will be shed for you” (Luke 22:19-20).
The technical term the Church has used for many centuries now to describe what happens to the bread and wine is transubstantiation. It happens during the great Eucharistic Prayer of the Mass. The official teaching is phrased this way: “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.” (You can read all about this in nos. 1373-1377 in your Catechism of the Catholic Church.)
Frankly, I think this is one of the key theological concepts which every Catholic ought to know. Make sure you can do more than just spell it correctly!