The liturgical life of the Church revolves around the sacraments, with the Eucharist at the center (National Directory for Catechesis, #35).
At Mass, we are fed by the Word and nourished by the Body and Blood of Christ. We believe that the Risen Jesus is truly and substantially present in the Eucharist. The Eucharist is not a sign or symbol of Jesus; rather we receive Jesus himself in and through the Eucharistic species. The priest, through the power of his ordination and the action of the Holy Spirit, transforms the bread and wine into the Body and Blood of Jesus. This is call 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. (CCC 1413)
“The Sacrament of Sacraments”
I am the living bread that came down from heaven; whoever eats this bread will live forever;…Whoever eats my flesh and drinks my blood has eternal life and…remains in me and I in him. (John 6:51, 54, 56)
Anyone who desires to receive Christ in Eucharistic communion must be in the state of grace. Anyone aware of having sinned mortally must not receive communion without having received absolution in the sacrament of penance. (CCC 1415)
The Church warmly recommends that the faithful receive Holy Communion when they participate in the celebration of the Eucharist; she obliges them to do so at least once a year.(CCC 1417)
Receiving the Eucharist changes us. It signifies and effects the unity of the community and serves to strengthen the Body of Christ.