What is meant by "the communion of saints" in the Apostles' Creed? By "the communion of saints" is meant the union of the faithful on earth, the blessed in heaven, and the souls in purgatory, with Christ as their Head.
Through the communion of saints, what can the blessed in heaven do for the souls in purgatory and the faithful on earth? Through the communion of saints, the blessed in heaven can help the souls in purgatory and the faithful on earth by praying for them.
It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from their sins. (II Maccabees 12:46)
Further reading: CCC 946-948