I found this site has a nice explanation:
franciscanmedia.org/presentation-of-mary/.
Also, in researching it, I found many scriptural references to being dedicated to the Lord which most certainly apply to Our Lady, for she didn’t decide she’d give her life unreservedly to God just at the Annunciation, but well before that. We can see this in her reply to the Archangel Gabriel when she asked, “How can this be since I know not man?” If she had expected to have normal sexual relations with her husband her more logical answer would have been, “Then Joseph is to be the father, seeing that I’m betrothed to him.” Gabriel cleared up everything for her by telling her what she’d already decided–that she was set apart for God alone, when he told her that “the power of the Most High will overshadow you and you will conceive and bear a son.” This was the same Power that overshadowed the Tabernacle of the Lord which God had Moses construct. There are many parallels between Mary and the Ark of the Covenant and other such biblical iconography. St. Alphonsus de Ligouri’s book “The Glories of Mary” is a great scriptural resource for the Marian teachings and feast days, as well.