Today's feast celebrates Mary as the Mother of God.
This title is Mary's primary one, the reason for her other gifts of grace.
The Council of Ephesus in 431 gave her this title, in response to the heresy of Nestorius. His error was really about Christ. Nestorius said that Jesus Christ was two persons, a divine person and a human person. But the problem with this is that it divides Jesus.
Instead, the Church teaches the unity of his person: Jesus Christ is one divine person with two natures, divine and human. He is fully human and fully divine, but both divine and human natures fully belong to the Son, the second Person of the Trinity, who became Incarnate.
Because of this, we can truly say that Mary is the Mother of God.
The people of Ephesus probably didn't follow all the theological subtleties of the debate. But they knew that they had always called Mary the Mother of God. So they recognized the error in Nestorius' teaching, even though they didn't have academic training.
That holds a lesson for us today. There is that "sense of the faith" that the members of the Church have. The laity have often held on to the true faith even when erroneous teachings were getting popular.