It is the Church, but with a broader sense.
What is the meaning of communion? This word is the union of two words: common and union. It is then, one common union, unity of something in common.
Who are the Saints? They are the Christians, the members of the church. This is what Saint Paul confirms when he greats the Colossians in Col 1, 2 saying “to God’s holy people in Colossae, our faithful brothers in Christ.” He also repeats this in Philippians 4, 21-22 saying “My greetings to every one of God’s holy people in Christ Jesus. The brothers who are with me send you their greetings. All God’s holy people send you their greetings.”
…What is the Church if not an assembly of all saints? The Communion of Saints is precisely the Church (Catechism 946). among other realities.
There is a common union across all Christians, what joins every one of the members of the church is primarily the sacrament of baptism,
Here, all saints are baptized in general and they are called to live in holiness, called to be saints in heaven.
The communion of saints is the common union of Jesus Christ as head of the Church, and all of its members and the union of the later between themselves.
This communion, thanks to the baptism, is not only among those that are in pilgrimage in this world, but also with those souls that are in purgatory and in heaven.
All these Christians are members of the Church that has three states: The Church Militant (here on Earth), the Church Penitent and the Church Triumphant (Catechism 954) these are not three separate churches but one: The Holy Catholic Church
Among all these saints there is a wealth of spiritual goods: those on earth honor those in heaven and seek their intercession, they pray for those that are being purified. Those in heaven pray and intercede for all the members of the Church.
The term “communion of saints” therefore has two closely linked meanings: communion in holy things sancta)” and “among holy persons (sancti) (Catechism 948).