The CCC says this about it:
1396 The unity of the Mystical Body: the Eucharist makes the Church. Those who receive the Eucharist are united more closely to Christ. Through it Christ unites them to all the faithful in one body - the Church. Communion renews, strengthens, and deepens this incorporation into the Church, already achieved by Baptism. In Baptism we have been called to form but one body.230 The Eucharist fulfills this call: "The cup of blessing which we bless, is it not a participation in the blood of Christ? the bread which we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread:"231
If you are the body and members of Christ, then it is your sacrament that is placed on the table of the Lord; it is your sacrament that you receive. To that which you are you respond “Amen” (“yes, it is true!”) and by responding to it you assent to it. For you hear the words, “the Body of Christ” and respond “Amen.” Be then a member of the Body of Christ that your Amen may be true.232
1397 The Eucharist commits us to the poor. To receive in truth the Body and Blood of Christ given up for us, we must recognize Christ in the poorest, his brethren:
You have tasted the Blood of the Lord, yet you do not recognize your brother,… You dishonor this table when you do not judge worthy of sharing your food someone judged worthy to take part in this meal… God freed you from all your sins and invited you here, but you have not become more merciful.233
1398 The Eucharist and the unity of Christians. Before the greatness of this mystery St. Augustine exclaims, "O sacrament of devotion! O sign of unity! O bond of charity!"234 The more painful the experience of the divisions in the Church which break the common participation in the table of the Lord, the more urgent are our prayers to the Lord that the time of complete unity among all who believe in him may return.
1399 The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. “These Churches, although separated from us, yet possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy.” A certain communion in sacris, and so in the Eucharist, "given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged."235
*
1400 Ecclesial communities derived from the Reformation and separated from the Catholic Church, "have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders."236 It is for this reason that Eucharistic intercommunion with these communities is not possible for the Catholic Church. However these ecclesial communities, "when they commemorate the Lord’s death and resurrection in the Holy Supper . . . profess that it signifies life in communion with Christ and await his coming in glory."237*
1401 When, in the Ordinary’s judgment, a grave necessity arises, Catholic ministers may give the sacraments of Eucharist, Penance, and Anointing of the Sick to other Christians not in full communion with the Catholic Church, who ask for them of their own will, provided they give evidence of holding the Catholic faith regarding these sacraments and possess the required dispositions.238
212 St. Ambrose, De Sacr. 5, 2, 7: PL 16, 447C.
213 St. Ambrose, De Sacr. 4, 2, 7: PL 16, 437D.
214 Roman Missal, EP I (Roman Canon) 96: Supplices te rogamus, omnipotens Deus: iube haec perferri per manus sancti Angeli tui in sublime altare tuum, in conspectu divinae maiestatis tuae: ut, quotquot ex hac altaris participatione sacrosanctum Filii Corpus et Sanguinem sumpserimus, omni benedictione caelesti et gratia repleamur.
231 ⇒ 1 Cor 10:16-17.
232 St. Augustine, Sermo 272: PL 38, 1247.
233 St. John Chrysostom, Hom. in 1 Cor. 27, 4: PG 61, 229-230; cf. ⇒ Mt 25:40.
234 St. Augustine, In Jo. ev. 26, 13: PL 35, 1613; cf. SC 47.
235 UR 15 # 2; cf. ⇒ CIC, can. 844 # 3.
236 UR 22 # 3.
237 UR 22 # 3.
238 Cf. ⇒ CIC, can. 844 # 4.