In short, communion sanctorum, means the “communion of the saints”. There are different views on what the “communion of the saints” means between the Catholic and Protestant church.
This belief is often over emphasized in the Catholic and Orthodox churches. It is taken to mean that the church is comprised of a spiritual communion or fellowship of all saints, including those living (ecclesia militans “the church militant,” 1 Cor. 12:1) and those dead (ecclesia triumphans “the church triumphant,” Heb. 12:1). In such a fellowship, the body of Christ continues to find strength in all its members.
Protestants often find offense with this view and at the concept of this belief because of the abuses of the Catholic and Orthodox churches who advocate prayer to dead saints as a benefit of such communion. Protestants also do not take favor with the Catholic’s Church’s unbiblical teachings on the “communion of the saints” when connected with mysticism, which is prominent in Catholicism.
Protestants reject such doctrines as purgatory and the intercession of saints (i.e. praying to Mary) which the Catholic Church connects with the “communion of Saints”. Protestants take a biblical view of the communion of the saints as stated in the Apostle’s Creed, which clearly defines a proper view of this teaching.
The father of the Reformation movement, Martin Luther defines communion sanctorum as follows:
"The communion of saints." This is of one piece with the preceding ["the holy catholic church"]. Formerly it was not in the creed. When you hear the word "church," understand that it means group [Haufe], as we say in German, the Wittenberg group or congregation [Gemeine], that is, an holy, Christian group, assembly, or, in German, the holy, common church, and it is a word that should not be called "communion" [Gemeinschaft], but rather "a congregation" [eine Gemeine]. Someone wanted to explain the first term, "catholic church" [and added the words] communio sanctorum, which in German means a congregation of saints, that is, a congregation made up only of saints. "Christian church" and "congregation of saints" are one and the same thing.
 Luther, "Sermons on the Catechism," 1528. Reprinted in Martin Luther: Selections from his Writings, John Dillenberger ed. p. 212.