How does the Orthodox Church explain this?
Those Church Fathers who have examined the Book of Revelation look at it in three ways:
1) Like most Jewish-Christian apocalyptic literature from the Hellenistic period on up to the end of the 1rst century, it is speaking against the pagan regime under which the faithful suffered and calling out those members of the faithful who capitulate to said regime. Remember that "prophecy" has two meanings: (a) foretelling the future through divine inspiration; and (b) announcing the will/judgment of God boldly. The Book of Revelation obviously accomplishes the latter (and that is usually said to be its main focus); and it's been interpreted to have predicted events that have already occurred, mainly many, many centuries ago. I suggest checking out the Orthodox Study Bible, which has plenty of more info along these lines.
2) Doctrinally, its focus is Christology (not predictions about this or that event). Here's a quote from His Grace Bishop Gerasimos of Abydos of blessed memory:
The fundamental idea of the Book of Revelation is the belief that Christ is the Lamb, sacrificed for us. He who opens the seals of the book is the ruling king revered and worshiped by the Church in heaven and on earth (Rev. chs. 4, 5, 6, 12, 19). The Lord Christ, the sacrificed Lamb, leads the struggle with His armies, the Saints of the Church. Their weapon is their faith in the word of God to the point of self-sacrifice. The blood of the Lamb, the sacrifice of Christ on the Cross and His Resurrection strengthen them; the victory over the enemy is certain (Rev. 6:2; 12:11; 19:17-18). The Christology in the Book of Revelation, which emphasizes Christ as the sacrificial Lamb, is the most powerful witness for the power of the Cross in the salvation of the world, as the Church experienced it. The Cross, the Resurrection, the Parousia, all indicate clearly the absolute sovereignty of Christ (cf. 1 Cor. 11:26).
In other words, statements about Christ's imminent return are really statements about his current
power to save souls and right wrongs. Christ's power is total and complete -- even now -- despite the fact that he hasn't yet fully returned in history. That's what theologians call the "in-breaking of the Kingdom", which we experience in His Body, the Church. Christ's Kingdom is "breaking in" to history even now through the Church; this "breaking in" is paradoxical: one the one hand, it is a foretaste; on the other, it is full and complete and powerful. Anyway, the point is that Christ conquers all and rules over all.
3) Other Fathers focus on its liturgical elements. Its vision of heaven is the same reality -- and structure -- that we experience in the Divine Liturgy.