For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread:
And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me.
After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me.
For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come.
(1 Cor. 11:23-26)
Holy Communion also known as the sacrament of the Eucharist, is one of the seven sacraments.
For Roman Catholics, it is the most important sacrament. "The Eucharist is 'the source and summit of the Christian life.' 'The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate, are bound up with the Eucharist and are oriented toward it. For in the blessed Eucharist is contained the whole spiritual good of the Church, namely Christ himself, our Pasch.'" (Catechism of the Catholic Church)
Celebration of the holy communion is the reason for (in fact, it is synonymous with) the mass. For Catholics, the bread and wine offered by the priest is not merely symbolic of the body and blood of Jesus Christ, but through the miracle of transsubstantiation, are transformed into the actual body and blood of Christ.