Short answer: They’re not dead. (Mark 12:27; Luke 20:38)
Longer answer: What happens when we depart this life? The Orthodox Church believes that some will find themselves in the paradise of which our Lord Jesus spoke when He said to the repentant thief on the cross, “Today you shall be with me in paradise.” These are not in need of our prayers; but other than those who have been recognized by the Church as saints, we do not know who they are, and so we offer prayers for them. As for the rest of the departed, they find themselves in a state of suffering as they await the time when the Lord will “come again to judge both the living and the dead” (as we recite in the “Symbol of Faith,” as the Nicaean Creed is called). On that great and terrible Day of Judgment, the final disposition of every human soul will take place: some (those on the Lord’s right hand) will enter into the Kingdom, while others (those on His left hand) will depart into torment without end. Some of those entering the Kingdom at that time will be those who are now enjoying paradise; but some will be from the group of those now undergoing torment: and these are the people for whom we offer prayers, both privately and as the Church. Part of the reason for this arises from visions which several saints were given, showing that the prayers offered in this life are effective on behalf of those who departed this life, either in reducing the level of suffering, or its duration, with some even being set free to enter paradise from hades (the name generally given to the temporary place of torment).
Another part of the reason for this practice rests on the biblical statements such as the ones cited above, and the understanding that “God desires not the death of a sinner, but that he turn from his ways, and live.” (Ezekiel 33:11, as quoted in the Prayer of Absolution at the end of one’s confession.) Just as we pray for one another in this, the land of the living, trusting that such prayers are often helpful for those mentioned in these prayers (such as for healing, or for deliverance from difficult circumstances), so we believe that our prayers can also be helpful to those who have departed this life who are not yet in “a place of light, a place of green pasture, a place of repose, from which all sickness, sorrow, and sighing are fled away,” as we pray in the Funeral Service and in the memorial services offered for the departed.