A - Thanks for the great question! First let us explore what it means to be "present". Vatican II in Sacrosanctum Concilium (the Constitution on the Sacred Liturgy) says that Christ is "present" to us in five different ways. I will highlight them in the text below from SC, 7:
To accomplish so great a work, Christ is always present in His Church, especially in her liturgical celebrations. He is present in the sacrifice of the Mass, not only in the person of His minister, "the same now offering, through the ministry of priests, who formerly offered himself on the cross" (20), but especially under the eucharistic species. By His power He is present in the sacraments, so that when a man baptizes it is really Christ Himself who baptizes (21). He is present in His word, since it is He Himself who speaks when the holy scriptures are read in the Church. He is present, lastly, when the Church prays and sings, for He promised: "Where two or three are gathered together in my name, there am I in the midst of them" (Matt. 18:20) .
So, we have Christ truly present:
- In the priest,
- In the Sacraments,
- In the Scriptures,
- In the people,
- Especially in the Eucharist.
The Catechism tells us more about how Christ is present to us:
CCC 1374 The mode of Christ's presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as "the perfection of the spiritual life and the end to which all the sacraments tend." In the most blessed sacrament of the Eucharist "the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained." "This presence is called 'real' - by which is not intended to exclude the other types of presence as if they could not be 'real' too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present."So, what the Church is saying is that Jesus isn't only present in a spiritual way. But, rather there is a substantial change and all of Christ is now here for us.
In Benedict's first encyclical he writes on this topic:
“Mysterium fidei! - The Mystery of Faith!”. When the priest recites or chants these words, all present acclaim: “We announce your death, O Lord, and we proclaim your resurrection, until you come in glory”.
In these or similar words the Church, while pointing to Christ in the mystery of his passion, also reveals her own mystery: Ecclesia de Eucharistia. By the gift of the Holy Spirit at Pentecost the Church was born and set out upon the pathways of the world, yet a decisive moment in her taking shape was certainly the institution of the Eucharist in the Upper Room. Her foundation and wellspring is the whole Triduum paschale, but this is as it were gathered up, foreshadowed and “concentrated' for ever in the gift of the Eucharist. In this gift Jesus Christ entrusted to his Church the perennial making present of the paschal mystery. With it he brought about a mysterious “oneness in time” between that Triduum and the passage of the centuries.
The thought of this leads us to profound amazement and gratitude. In the paschal event and the Eucharist which makes it present throughout the centuries, there is a truly enormous “capacity” which embraces all of history as the recipient of the grace of the redemption. This amazement should always fill the Church assembled for the celebration of the Eucharist. But in a special way it should fill the minister of the Eucharist. For it is he who, by the authority given him in the sacrament of priestly ordination, effects the consecration. It is he who says with the power coming to him from Christ in the Upper Room: “This is my body which will be given up for you This is the cup of my blood, poured out for you...”. The priest says these words, or rather he puts his voice at the disposal of the One who spoke these words in the Upper Room and who desires that they should be repeated in every generation by all those who in the Church ministerially share in his priesthood. (EE, 5)
So, how is Christ present is best answered by saying that it is a "real presence" of the "whole Christ" in his "body, blood, soul and divinity".
What a great thing to meditate on more!
I hope this helps.