Those Christians who believe in the Real Presence of Christ in the Eucharist at Communion do not consider the Communion Elements to be a mere symbol of Christ's Body and Blood. They assert that the elements are actually transformed into the Body and Blood of Christ.
Belief in this transformation depends on the belief that the observable qualities of a thing such as color, texture, weight, etc. can be separated from the essential nature of the thing itself. The observable qualities are referred to technically as accidents and the essential nature is referred to as the substance.
In the theory of Transsubstantiation, the accidents of the Communion Elements remain the same while their substance is wholly transformed into that of the Body and Blood of Christ. Roman Catholics, most prominently, subscribe to this theory.
In the theory of Cosubstantiation, the accidents of the Communion Elements remain the same while the substance of the Body and Blood of Christ are added in along with the substance of the elements. Lutherans, for example, subscribe to this theory.
Regardless of the theory, exactly how this transformation is affected is held as a mystery by those who subscribe to the belief in the Real Presence of Christ in communion.