What is happening here
On Sundays and solemnities, after the homily, the assembly stands and recites together the Creed — the formal statement of what the Catholic Church believes. The most commonly used Creed is the Nicene Creed, hammered out in the great ecumenical councils of the fourth and fifth centuries to articulate the faith against various early heresies. The Apostles' Creed, an older and shorter formulation, is also permitted, especially during Lent and Easter or at Masses with children.
To pray the Creed at Mass is a serious act. The assembly is not voicing a personal opinion or a private spiritual journey; it is professing the faith of the universal Church, in the words the Church has settled on after centuries of theological reflection. To say "I believe" together with the rest of the assembly is to step into a conviction larger than oneself — to be carried by the faith of the Church in those moments when one's own faith feels small.
Specific moments in the Creed call for particular reverence. At the words "and by the Holy Spirit was incarnate of the Virgin Mary, and became man," the assembly bows — a small physical gesture that acknowledges the central mystery of Christian faith, that God himself entered our world as one of us. On Christmas and on the Annunciation (March 25), the assembly genuflects at the same words, marking the unrepeatable wonder of the Incarnation.
The Creed is omitted at weekday Masses (because the Liturgy of the Word is already shorter on weekdays) and at most weddings and funerals (which use a different ritual structure). When it is recited, it stands as one of the most theologically dense moments of the entire Mass.