As we all know perfectly well, the Church never defines anything about the Blessed Virgin Mary unless it tells us something about Jesus or something about us. [this is why, short of a new heresy arising about why we cannot pray for each other, the Maria Mediatrix Coredemptrix people are going to be greatly disappointed.] So, what is Holy Mother Church setting out to teach us by defining the content of this feast as doctrine? What does the Dormition, alias the Assumption, mean for us believers?
First, a proper definition of this feast: Mary, the mother of our Lord Jesus Christ, at the end of her life on earth, was taken up to heaven by her Son, both soul and body together. There she lives forever in the glory of the Presence of God in her resurrection body.
Note that she did not not die --- though that would not have been out of line, since Enoch did not die, nor Elijah, and maybe not Moses either. She did die, and her Son resurrected her, and took her to Heaven.
This is not so different than what has been promised to each one of us. Dormition, falling asleep in the Lord, will come for each of us. We are made "to be happy with Him forever and ever in heaven", as the first grade CCD book taught us. And just as His mother Mary has been resurrected to dwell in heaven body and soul together, the time will come when He will raise us up, and our soul will rejoin our resurrection body to the praise and glory of God.
Mary is the first-fruit of the general resurrection. Where and as she is, so shall we come to be, whole people, body and soul together, happy with God our Creator and Father in His eternal presence.