explanation of the Mass

Through His sacrifice on the Cross, Christ conquered sin and death once and for all. So why do Catholics seemingly sacrifice Christ over and over again at Mass?
I'm sure many of us have heard this question at one time or another. The key is to understand that the sacrifice of Christ on the Cross and the sacrifice of the Mass are the same, single sacrifice. Christ’s sacrifice on the Cross brought about “objective” redemption—in other words, the possibility of salvation for all people of all time. No other sacrifice is needed.
The sacrifice of the Eucharist, commonly called “the Mass,” does not repeat or multiply this sacrifice, then, but rather makes present this one, definitive sacrifice of Christ here and now in our local parish church. This brings about what we call “subjective” redemption—in other words, the application of the merits of Christ’s sacrifice to individual men and women.
In continually offering the sacrifice of the Mass (cf. Mal. 1:11), the Church fulfills the command of the Lord Jesus to “do this in memory of me” (Lk. 22:19). Through the sacrifice of the Mass, we are able to partake of the Body and Blood of Christ, without which we have no life in us (cf. Jn. 6:53-56).
For more information on this topic, see Catechism of the Catholic Church, nos. 1362-72. I also highly recommend my friend Thomas Nash’s book, Worthy is the Lamb, on the biblical roots of the Mass.