The Catechism in Questions & Answers

503. What happens when the wafers are consecrated?

The body and blood of Jesus Christ become present in the consecration.
Bread and wine are not changed in their substance through the consecration. In other words, the bread and wine are not transformed. Rather, the substance of the body and blood of Jesus is joined to the bread and wine. This event is described as ‘consubstantiation’.
In Holy Communion, bread and wine are not mere images or symbols for the body and blood of Jesus. Rather, the body and blood of Jesus Christ are truly present after the consecration.

Symbol: see explanation of Question 498.

The term ‘consecration’ is derived from the Latin word consacrare, which means “to dedicate” or “to sanctify”. The term is used in the sense of “dedication” when bread and wine are consecrated for Holy Communion.
The word ‘substance’ derives from the Latin word substantia, which means “essence”, “constitution”, or “composition”. It thus describes what something consists of.