@Ryan since I have not seen it in the replies so far, I will state the easy and obvious answer, it is the Body and Blood of out Lord and Savior Jesus Christ.

There is of course 2000 years of theologians unpacking what that means, but I think that is where we start, and to be honest most Sunday mornings when I receive the Eucharist, this is enough.

"For I received from the Lord what I also handed on to you, that the Lord Jesus on the night when he was betrayed took a loaf of bread, and when he had given thanks, he broke it and said, ‘This is my body that is for you. Do this in remembrance of me.’ In the same way he took the cup also, after supper, saying, ‘This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.’ For as often as you eat this bread and drink the cup, you proclaim the Lord’s death until he comes." 1 Cor 11:23-26 (NRSV)