I'm going to attempt to explain the Catholic understanding of the Eucharist with a bit of hesitation. Some of this gets a bit abstract and unless you have a bit of training in aristotlean-thomistic philosophy can go over people's head but here goes for those interested.
This is only an attempt at explain what it is and not what relationship we should have with Him. For if Catholics are right, the type of relationship we should have..........should be obvious.
First off, it's important to clarify this first, there is no tangible test (weight, chemical composition or properties) that can detect a difference before and after the consecration (the act of turning it into body and blood). The only exception to this is Eucharistic miracles.
There is however a "material" change in the sense that what was once bread and wine ceases to be bread and wine. There is what the common mind thinks of as "physical" and "material," and then there is what a philosopher means by them. Both the matter and form of the bread and wine are no longer there in transubstantiation (once again, what you call communion); they are replaced with the matter and form of Christ. So it would not be proper to say wheat, carbon, and the entirety of what makes up bread and wine or anything of that sort is still there. The accidents, however, remain (the bread and wine). When the accidents disappear (whether through eating) the real presence ceases. What exactly happens after that is open to discussion and has never been settled by catholic thinkers.
By accident we do not mean material parts. It’s a language limitation with the English. Modern man use physical and material interchangeably. By accidents we mean anything which exists only in something, and unable to exist outside of it. Like color or size, etc. You cannot have blue existing by itself, there must be something that is blue. And so on.
To expand further; Christ's proper place, as we all know is heaven. His body is physically present there. If He were to be physically present here with us, it would mean that all 6’-0” 175 lbs (just a guess) of Him taking up that space here on earth is right here and not in heaven. It does not quite work that way.
Rather, it means (as noted above) that the matter and form of bread become His body (but not physically in that He does not subsume the physical accidents of the bread nor does He move from heaven). Rather, His body and His body alone are made present by the force of the sacrament and, since they are necessarily joined to His Body, His soul, divinity, blood, etc is present with it. But not His accidents as those are separable.
I know this gets abstract but I didn’t know how else to talk about it without getting philosophical.