The Catholic teaches infallibly, and has taught infallibly since the preaching of St. Paul, that the bread and wine of thanksgiving sacrifice truly become the Body and Blood of Christ in substance, while remaining under the appearance of bread and wine.  

When Father says, "this is my body", the bread becomes the true flesh of Christ.  When Father says, "this is my blood", the wine becomes the precious blood of Christ.  It is not symbolic.  Nor is Christ only present next to the bread and wine:  He is literally, physically present on the altar, while the bread and wine have ceased to exist.  This is why, when we approach for Communion and the minister announces to us "the body of Christ", we reply "AMEN"; for we are in agreement that it is no longer bread, no longer wine, but the true body and blood of our Savior with whom we commune.  
 

"Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him."
John 6:53-56
 

Then he took the bread, said the blessing, broke it, and gave it to them, saying, “This is my body, which will be given for you; do this in memory of me.”
Luke 22:19

1324 The Eucharist is "the source and summit of the Christian life." ... For in the blessed Eucharist is contained the whole spiritual good of the Church, namely Christ himself, our Pasch."

1325 "The Eucharist is the efficacious sign and sublime cause of that communion in the divine life and that unity of the People of God by which the Church is kept in being. It is the culmination both of God's action sanctifying the world in Christ and of the worship men offer to Christ and through him to the Father in the Holy Spirit."

1326 Finally, by the Eucharistic celebration we already unite ourselves with the heavenly liturgy and anticipate eternal life, when God will be all in all.

1333 At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, become Christ's Body and Blood ... The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ;

1337 The Lord, having loved those who were his own, loved them to the end...In order to leave them a pledge of this love, in order never to depart from his own and to make them sharers in his Passover, he instituted the Eucharist as the memorial of his death and Resurrection, and commanded his apostles to celebrate it until his return; "thereby he constituted them priests of the New Testament."

The Church teaches that Christ is entirely present in each particle of the Host, each drop of the Precious Blood.  Because of this reality, it is of the utmost importance that we approach Communion in an attitude of reverence and care that prevents any unforseen desecration or mishap with the Body and Blood.  
 

• If you are aware of any mortal sin on your soul, or if you are not a Catholic and initiated into the sacrament of Holy Communion, you may not receive.  Those who cannot receive the Host or Precious Blood may present themselves for a blessing from either a deacon or a priest.  Extraordinary ministers should not be approached for blessing, children or otherwise. 
• Communion may be received either on the tongue or by hand.  If you receive in the hand, please do not "pluck" the Host from the minister's hand, and do not return with it, unconsumed, to your pew.  The Host must be consumed upon receiving it.
• Recipients will bow or genuflect in reverence, prior to receiving the Host.  
• If the Host falls to the floor while you are receiving, please retrieve it and give back to the minister who will either consume it afterwards or have it properly dissolved in water.
• If a drop of the Precious Blood falls to the floor, do not attempt to wipe it away.  The deacon or extraordinary minister should be made aware, and they will take proper steps to clean the area immediately, per the requirements of the Church.  
• Recipients may received Communion in either a standing or kneeling posture.  It is not licit to deny Communion to someone who wants to receive in a kneeling posture.  
• Communion may be received no more than twice in a single day.