In the Roman Catholic Mass, the essential words of consecration, spoken by the priest in the person of Christ, are: “This is my body” (when holding the bread) and “This is my blood” (when holding the chalice). These words, along with the accompanying actions, are believed to change the bread and wine into the Body and Blood of Christ. 

Here’s a more detailed explanation:

• The Words of Institution:The words of consecration are also known as the Words of Institution. They are derived from the Last Supper narrative in the Gospels. 
• In Persona Christi:The priest speaks these words in the “person of Christ” (in persona Christi), meaning he is acting in the place of Christ. 
• Essential for Validity:The Catholic Church teaches that these words are essential for the consecration to be valid. If the priest omits or significantly alters these words, the consecration is considered invalid. 
• Beyond the Words:While the words are crucial, the consecration also involves the priest’s intention to do what the Church intends, and the use of proper matter (wheat bread and grape wine). 
• Transubstantiation:The Catholic Church believes that the bread and wine, through the power of the Holy Spirit and the words of consecration, undergo a complete change of substance into the Body and Blood of Christ, a doctrine known as transubstantiation.