The Eucharist is a Sacrament in the Catholic Church by which bread and wine become the Body and Blood of Jesus Christ. It is important to understand that the Eucharist is itself Christ. The substance of the bread is Christ Himself and the appearance, which is called the species, remain the same. So when a person enters a Catholic Church and they see a consecrated Host, they see a piece of bread (species) which is actually Christ Himself (substance)