The Lord's Supper:

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​Luther retained the sacrament of the Lord's Supper, which is the central act of worship in the Lutheran denomination. But the doctrine of transubstantiation was rejected. 

While Lutherans believe in the true presence of Jesus Christ in the elements of bread and wine, the church is not specific in how or when that act occurs. Thus, Lutherans resist the idea that the bread and wine are mere symbols.

Transubstantiation is the official Roman Catholic teaching referring to a change that takes place during the sacrament of Holy Communion (Eucharist). This change involves the whole substance of the bread and wine being turned miraculously into the whole substance of the body and blood of Jesus Christ himself.
During the Catholic Mass, when the Eucharistic elements -- the bread and the wine -- are consecrated by the priest, they are believed to be transformed into the actual body and blood of Jesus Christ, while keeping only the appearance of bread and wine.
Transubstantiation was defined by the Roman Catholic Church at the Council of Trent:
"... By the consecration of the bread and wine there takes place a change 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. This change the holy Catholic Church has fittingly and properly called transubstantiation."