That is God. Jesus Christ is there present, in his full body, blood, and soul. He is present physically, in his full humanity and divinity. Through the powers bestowed upon the priest in his Ordination, what was ordinary bread and wine at the beginning of Mass is transformed into the physical flesh and blood of Jesus during the Consecration. Though it still maintains the appearance, taste, and texture of bread and wine (the "accidents" of bread and wine, as defined by Church doctrine), it has been completely transformed into Jesus' body and blood.
I receive God every week. Every time I go to Mass. For a time, I'm a tabernacle, because until the last particle of the Host dissolves, I'm holding Jesus Christ inside me. What thing on this earth can ever be greater than that? It's... almost incomprehensible. Jesus Christ has, for mankind, taken on the appearance of bread and wine so that he can be with us in the fullest way possible. Apart from Creation, or the Redemption, this is God's greatest gift to humankind. He's given (and giving) Himself.
I asked the question a while back on a previous post: "Why am I a Catholic?". Well, even if I hadn't already reasoned that the Catholic Church is the ONE, TRUE, HOLY, and APOSTOLIC Church, that the Catholic Church has the Truth in fullness and entirety, and that the Catholic Church is the one and only Church that Jesus Christ founded - I'd be a Catholic because the Catholic Church has more than any other Church has. It has God. And I can worship Him, and touch Him, and see Him every single time I go to Mass.