What is communication of saints?

I believe you mean the Communion of Saints. It is defined as :
"The communion of saints (in Latin, communio sanctorum), when referred to persons, is the spiritual union of the members of the Christian Church, living and the dead, those on earth, in heaven, and, for those who believe in purgatory, those also who are in that state of purification.

They are all part of a single "mystical body", with Christ as the head, in which each member contributes to the good of all and shares in the welfare of all."