John *saw* a Great Sign: a woman clothed with the sun. It does not mean that this woman was in the middle of a vast thermonuclear reaction surrounded by solar hydrogen fusion energy. It just means this is what he saw: a woman clothed with the sun. It's symbolic, but it's also literally what he saw.
I should have clarified that the "first" meaning of this passage would have to be what was in the plain text itself: that the woman is the mother of the Messiah: therefore Mary. Other meanings are derivative from that.
The passage strongly reflects the maternal nature of the Church, because Mary is, in a sense, the first embodiment of the Church. Mary/Church "hears the Word of God and keeps it." Christ comes and dwells in Mary/Church. Mary/Church carries Him in her wherever she goes. Mary/Church brings Christ into the world. She is with Him from beginning to end: from His conception, through His birth, through His growth in His human nature, faithfully following Him even to the foot of the Cross; present to receive the Holy Spirit again at Pentecost.
And she has this ongoing maternal association with the Body of Christ even in what you could call the ecclesial sense:
Revelation 12:17 "Then the dragon became angry with the woman and went off to wage war against the rest of her offspring, those who keep God’s commandments and bear witness to Jesus."
We -- you, me, all the believers --- are the Church, the Body of Christ, and Mary is likewise our mother, because we are "the rest of her offspring."
Revelation 12 is very, very rich with meaning, and repays careful reading.
