Sin. Sometimes (okay, frequently) we miss the mark - that’s literally what the Greek word for sin, hamartia, means - we fall tragically short of the purpose for which God created us and in so doing we separate ourselves from Him. From our perspective, it is knowing full well what God wants us to do, and then not doing it, or worse - doing the opposite. We almost can’t help it, and we attribute this to ancestral sin (which is generally preferred over “original sin” in Orthodoxy.
As we are all descendants of Adam we inherit the consequences of his sin (NB: we do NOT believe as the West does, that we are also guilty of the sins of Adam and Eve), the most obvious of which is physical death in this world. St. Gregory Palamas tells us that due to Adam’s disobedience, the image of man is forever tarnished and disfigured. St. Gregory of Nyssa tells us that in so doing, we are doing damage to the likeness of God, and that our only hope for salvation is through our baptism and reliance on the incalculable Grace of God and in regular participation in the Holy Mysteries of His Church.