Let's start with the death we all know for sure: physical death. The particular order that keeps the body alive reaches a certain point that can no longer be sustained and is lost. The human being, or animal, or plant dies.
This physical death is related to the more general tendency of the universe toward entropy. Everything dissipates, everything is passing away right now. The great question for materialists is, in fact, how come in such a universe life appears at all. Their answer is that it is "just" statistics, but that is not enough.
Orthodox Christianity teaches us that the Universe was created without entropy. People and things were so close to God and there was no tendency away from Him, just being in Him, and therefore always being "energized" by the Spirit of Life Himself.
The devil then seduced Adam and Eve to break this harmony. They gave steps in the direction opposite to God, thus away from life.
Therefore they did not have to die immediately after disobeying, but, specially after refusing to repent, they were already "floating away" from life, thus, in a certain way they were "dead men walking". As we all are.
Jesus incarnation, among other things, reverted all this. His healings, His hints of a new Creation and finally His own resurrection showed that not only entropy/death could be reverted, but that it had done that. He promised us that we all - Christians and non-Christians will be resurrected in the end of this universe. He will not allow entropy to follow its full course. He will recreate the Universe as it was meant to be, with no entropy.
This is an expression of His infinite mercy. We are all saved from being completely destroyed by death, miraculously kept in existence, despite even the desintegration of our bodies.
"Second death" is metaphorical. No one will be destroyed in the literal sense, but we can now choose how we will live this eternity. Jesus never mentioned that you had to be Christian or Orthodox to be on the right side on Judgement Day. He is very clear about the criteria: those who love, who helped those in need, those who gave their lives for their neighbors, these will, after being resurrected be on his right side and the rest on his left side, that is , hell. Being condemned to an eternity of suffering in the place created to be the prison of demons is this "second death".
While Last Judgment will be the full expression of His Glory and Justice, the Church is still an expression of His mercy, accepting all, even the wicked. Nevertheless, the Church is that future vivifying reality in our present time, a foretaste of the good things to come, so anyone can taste it, be prepared by it, strengthened by it and choose. Many do not take advantage of this opportunity, wasting their participation in the Church, while others shed tears for not having the possibility of having this intimate relation with God.