What is the importance of Mass to Catholics?
They believe in transubstantiation, meaning they believe that the substance of the bread and wine become the literal Body and Blood of Christ.
It is a sacrament, meaning they receive Christ into their bodies and unites them with Christ and one another.
There are there aspects to any sacrament, and in Mass there is the Form (ritual actions), Matter (bread and wine), intention (the intention of the priest.)
The mass is a re-enactment of the sacrifice of Christ