A deep bow at the words about Christ taking flesh.
In the Creed, at "and by the Holy Spirit was incarnate of the Virgin Mary, and became man", bow deeply from the waist.
Hold the bow through the whole phrase and rise at "and became man".
On the Annunciation, March 25, and on Christmas Day, the whole congregation kneels for that line instead.
This is the hinge of the Creed and of history. God did not send a message, he took a body. Everything else Catholics believe about matter, the sacraments, the body, and the dignity of a person hangs on that sentence, so the body is asked to react to it.
The bow is medieval in origin and was a genuflection for a long stretch of Western practice, which is why kneeling survives on the two feasts of the Incarnation.
The current Missal kept the gesture and softened it to a profound bow on ordinary days, keeping the knee for the two days when the Church wants the moment felt in full.
The bow is in the rubrics of the Mass, so it is asked of everyone able to make it. The kneeling on March 25 and December 25 is likewise part of the rite. Anyone who cannot bow simply bows the head.