Some Christians believe that the bread and wine (or, grape juice) used in their communion services are simply that: bread and wine. They believe the action of sharing communion is a remembrance of Jesus. They are very uncomfortable with any talk of Jesus being present in the bread and wine.
Catholics (and the Orthodox churches) believe that the words Jesus used at the last supper were not simply metaphors. We believe that when he said, “this is my body”, he meant it. The compelling nature of the meal (his last on earth), and the need to feed his Church after his death and resurrection, required something more than symbols. So, the (very fair) question remains, How exactly is Jesus present in bread and wine?
Bread and wine are made of what Catholic philosophers describe as substance and accident. For example, bread is comprised of flour and water. Those are the accidents. A deeper reality exists in substance. “Breadness”, if you will. Think of substance as the sub soil of reality. How is it that we can identify bread in all of its multitude of appearances? The substance of bread, though unseen, enables the mind to grasp the concept of bread.
During the Eucharist, the accidents of the bread and wine do NOT change. It remains to the eyes and to the taste to be bread and wine. The deeper reality, the substance, is transformed into the body, soul, and divinity of Jesus. St. Thomas Aquinas wrote that Jesus comes to dwell in the consecrated bread and wine in the way your soul dwells in the body. (Full disclosure: while the Orthodox churches believe firmly in the real presence of Christ in the Eucharist, they are not comfortable with the Catholic philosophical explanation.)
There have been recorded miracles in which the host will bleed. This would be the presence of Christ permeating even the accidents of the bread. However, such events are very, very rare. The Real Presence is something you and I see with eyes of faith.