The teaching of the Church is clear—with the words of consecration, Christ becomes really and truly present among us. The substance of the bread and wine is changed into the substance of Christ’s body and blood. The accidents and appearances of bread and wine remain, however.
Eating and drinking the consecrated bread and wine (the body and blood of Christ) symbolize or sign our unity with Christ. Sharing in the one bread and cup further signs our unity with each other in Christ. God’s gift of the Eucharist is a sign of his care and providence just like the manna in the desert.
The bread and wine, offered at the offertory, are signs of Christ who will become present in the gifts and of his sacrifice being recalled and represented to the Father in the eucharistic celebration.