The life of Catholics revolves round the celebration of the sacraments with the Eucharist at the very centre. When we celebrate Mass we are fed with both the Word of God and the Body and Blood of Christ.
Catholics beleive that the Eucharist is really and truly the Body and Blood of Christ; that Jesus is truly present in the Eucharist under the form of bread and wine. The Eucharist is not merely a sign or a symbol: is is truly Jesus, body, blood, soul and divinity.
So Jesus said to them, “Very truly, I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Those who eat my flesh and drink my blood have eternal life, and I will raise them up on the last day; for my flesh is true food and my blood is true drink.
John 6:53-55
Through the power they receive at their ordination, and the working of the Holy Spirit, the preist offers bread and wine at Mass for it to be transformed into the Body and Blood of Christ. Catholics call this transubstantiation.
By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine Christ himself, living and glorious, is present in a true, real, and substantial manner: his Body and his Blood, with his soul and his divinity.
Catechism of the Catholic Church, 1413
On the night before he died, Jesus gathered his disciples around him to celebrate the the Passover. He gave the Jewish Passover new meaning and fulfilled the promises of the Old Testament. Jesus took, bread, broke it, and gave it to his disciples; likewise, he took a cup of wine and called it 'the blood of the covenant.'
When God made a covenant with the people of Israel at Mount Siani, the covenant was sealed with the blood of a lamb. The shedding of Jesus' blood on the cross has become for us the 'blood of the new covenant,' uniting us in an inseparable bond with God the Father. Jesus's sacrifice on the cross, of which the Eucharist is the memorial, establised a new relationship between God and the baptised, the new People of God.
In the same way the action of the Holy Spirit transforms the bread and wine into the Body and Blood of Christ, the Holy Spirit transforms those who reveive the Eucharist in the Body of Christ. In a very real sense, we become what we eat. Holy Communion, as the Eucharist is sometimes called, brings about and is the symbol of our union with each other and with God..
Mass is structured into two parts, called the Liturgy of the Word and the Liturgy of the Eucharist.
In the Liturgy of the Word we hear proclaimed to us readings from the Old Testament,the Prophet and the letters of the New Testament. We respond by singing or reciting the Psalms and then we listen to the Gospel. When the Word of God is proclaimed in Mass, Jesus, the Word of God, comes and teaches us himself, he is present in his Word and feeds and nourishes us. At the end of the Liturgy of the Word the priest gives a sermon or homily.
In the Liturgy of the Eucharist, bread and wine are brought forward to the altar where they are offered. As Jesus did at the Last Supper, the preist takes the bread, he blesses it, breakes it and gives it for the gathered people to eat. The same is done with the cup of wine, which becomes the Blood of Christ. The Eucharist is received by the faithful as food for their souls and strength to live the Christian life on earth.
At the end of Mass the prest blesses the people and they are sent our to share with everyone tyhe Good News they have heard proclaimed to them.