Our Lord Jesus Himself, true God and true man, is really and substantially present, in a mysterious way, under the appearances of bread and wine.
After the priest speaks the words of consecration, the appearances of bread and wine remain, meaning that bread and wine appear to be there but are not really there substantially. At the Last Supper, when our Lord gave the Apostles His Body, the Body looked and tasted like bread, though it was not bread; it was His Body, for He said so. When our Lord gave the Apostles His Blood, the Blood appeared to be wine and tasted like wine, though it was not wine; it was His Blood, for He said so.
The Holy Eucharist looks like bread and tastes like bread, but it is not bread; it is Jesus. To come to us, Jesus covers Himself with the appearances of bread and wine. Although we cannot understand this, we take the word of God that it is so. We have such belief in God, Who is all-truthful, that we believe all that He said about the Holy Eucharist. Therefore, the Eucharist is called a Mystery of Faith.
All Catholic priests have the power to change bread and wine into Christ. In the Eucharist, it is Jesus Himself Who consecrates through the priest as the words of consecration are said: “For this is my Body… For this is the chalice of my Blood, the Blood of the new and eternal covenant…”