Here is a great link that explains the Mass and answers your question:
catholic.com/thisrock/1990/9006chap.asp
A quote from this particular article is:
Frank Sheed summarizes Catholic teaching on the point in
Theology and Sanity:
“There is no new slaying of Christ in the Mass…Yet that it is the Christ who was slain upon Calvary is shown sacramentally by the separate consecration of bread to become His body and wine to become His blood. The essence of the Mass is that Christ is making an offering to the Father of Himself, who was slain for us upon Calvary. The Mass is Calvary, as Christ now offers it to His Father.”
The English word “Mass” comes from the concluding words of the Latin Liturgy "
Ite, missa est" which is roughly translated as "go, it is ended. The ancient Christians referred to the Mass as the Eucharist, or the Liturgy and you will often hear Catholic Christians doing the same today.
The first Mass was the Last Supper, celebrated by Jesus Himself.
I hope this helps you in your search for, and understanding of, the fullness of the faith!