What is it?
-> Council of Trent, Ch. 4: On Transubstantiation... "And because that Christ, our Redeemer, declared that which he offered under the species of bread to be truly his own body, therefore has it ever been a firm belief in the Church of God, and this holy Synod doth now declare it anew, that, by the consecration of the bread and of the wine, a conversion is made of the whole substance of the bread into the substance of the body of Christ our Lord, and of the whole substance of the wine into the substance of his blood; which conversion is, by the holy Catholic Church, suitably and properly called Transubstantiation."
-> "As regards transubstantiation, we care nothing about the sophistical subtlety by which they teach that bread and wine leave or lose their own natural substance, and that there remain only the appearance and color of bread, and not true bread. For it is in perfect agreement with Holy Scriptures that there is, and remains, bread, as Paul himself calls it, 1 Cor. 10, 16: The bread which we break. And 1 Cor. 11, 28: Let him so eat of that bread."
What do it give?
-> Council of Trent, Chapter 2: That the Sacrifice of the Mass is propitiatory, both for the living and the dead... "...this divine sacrifice which is celebrated in the mass, that same Christ is contained and immolated in an unbloody manner who once offered himself in a bloody manner on the altar of the cross; the holy Synod teaches, that this sacrifice is truly propitiatory, and that by means thereof this is effected, that we obtain mercy, and find grace in seasonable aid, if we draw nigh unto God, contrite and penitent, with a sincere heart and upright faith, with fear and reverence. For the Lord, appeased by the oblation thereof, and granting the grace and gift of penitence, forgives even heinous crimes and sins. For the victim is one and the same, the same now offering by the ministry of priests, who then offered himself on the cross, the manner alone of offering being different..."
-> The Condemnations of the Council of Trent/Trent-On the Sacrifice of the Mass, Chapter 3: "If any one saith, that the sacrifice of the mass is only a sacrifice of praise and of thanksgiving; or, that it is a bare commemoration of the sacrifice consummated on the cross, but not a propitiatory sacrifice; or, that it profits him only who receives; and that it ought not to be offered for the living and the dead for sins, pains, satisfactions, and other necessities; let him be anathema."
-> Luther's Small Catechism-VI: The Sacrament of The Altar, as the Head of a Family Should Teach It in a Simple Way to His Household: "What is the Sacrament of the Altar? - Answer: It is the true body and blood of our Lord Jesus Christ, under the bread and wine, for us Christians to eat and to drink, instituted by Christ Himself."
-> (continued from previous reference) "What is the benefit of such eating and drinking? - Answer: That is shown us in these words: Given, and shed for you, for the remission of sins; namely, that in the Sacrament forgiveness of sins, life, and salvation are given us through these words. For where there is forgiveness of sins, there is also life and salvation."
What is the benefit?
-> Catechism of the Catholic Church, 1391: "Holy Communion augments our union with Christ. The principal fruit of receiving the Eucharist in Holy Communion is an intimate union with Christ Jesus. Indeed, the Lord said: 'He who eats my flesh and drinks my blood abides in me, and I in him.' Life in Christ has its foundation in the Eucharistic banquet: 'As the living Father sent me, and I live because of the Father, so he who eats me will live because of me.'"