I can produce no finer summary than the following from a Synaxarion (a Byzantine liturgical book containing descriptions of various feasts and saints' days).
In accordance with the eternal purpose of God, who willed to prepare a most pure habitation for Himself in order to take flesh and dwell among men, Joachim and Anna were prevented from having children for many years. Their barren old age was symbolic of human nature itself, bowed down and dried up under the weight of sin and death, yet they never ceased begging God to take away their reproach. Now when the time of preparation determined by the Lord had been fulfilled, God sent an Angel to Joachim in solitude on a mountain, and to Anna in her affliction weeping in her garden, to tell them that the ancient prophecies were soon to be fulfilled in them: a child would be born to them, who was destined to become the veritable Ark of the new Covenant, the divine Ladder, the unburnt Bush, the living Temple where the Word of God would take up his abode. Through the conception of Saint Anna, the barrenness of human nature itself, separated from God by death, has on this day been brought to an end; and by the wondrous birth-giving of her who had remained childless until the age when women can no longer bear fruit, God announced and testified to the more astonishing miracle of the Conception without seed, and of the immaculate coming to birth of Christ within the heart and the womb of the Most Holy Virgin and Mother of God.
Even though the birth of the Blessed Virgin Mary took place through a miraculous action of God, she was conceived by the union of man and woman in accordance with the laws of our human nature, which has fallen through Adam's transgression and become subject to sin and corruption (cf. Gen. 3:16). As the chosen Vessel and precious Shrine prepared by God since the beginning of time, she is indeed the most pure and the most perfect of mankind, but even so, she has not been set apart from our common inheritance nor from the consequences of the sin of our first parents. Just as it was fitting that Christ, in order to deliver us from death by his own voluntary death (Heb. 2:14), should by His Incarnation be made like to men in all things except sin; so it was meet that His Mother, in whose womb the Word of God would unite with human nature, should be subject to death and corruption like every child of Adam, lest we not be fully included in Salvation and Redemption. The Mother of God has been chosen and preferred among all women, not arbitrarily, but because God foresaw that she would preserve her purity and keep it perfect: conceived and born like all of us, she has been worthy to become the Mother of the Son of God and the mother of us all. So, in her tenderness and compassion, she is able to intercede for us with her Son, that He may have mercy upon us.
Just as the Lord Jesus Christ was the fruit of the virginity of the holy Mother of God, so she herself was the fruit of the chastity of Joachim and Anna. And by following the same path of chastity we too, monks and Christian married people, can bring Christ to be born and grow in us.