See below, from https://thejesuitpost.org/2023/04/catholic-101-transubstanti... This seems clear enough to me, and can be summed up by saying that Catholics believe that they are literally eating the body of Christ.

... “transubstantiation” refers to a change in which the substance of a thing—what it really is—changes, while its physical characteristics do not. Of course, this sort of change only occurs in the Eucharist, which, though it appears to remain bread and wine throughout the Mass, nevertheless truly becomes the flesh and blood of Jesus.