One of the traditional rituals of the Catholic Church is the practice of genuflecting before the tabernacle as we enter and leave church. Genuflection (made by bending the right knee to the ground) is an acknowledgement of the presence of the Blessed Sacrament — the real presence of Christ — in the tabernacle. It helps us to pray with both body and soul.
In recent years, we have been encouraged to perform a similar ritual before taking Communion. We bow just before we receive the consecrated bread and wine to express our belief that they are the body and blood of Christ. During Mass, we celebrate the action of the Eucharist. First of all, Christ is seen to be present among the faithful gathered in His name; then in his Word, as the Scriptures are read and explained; in the person of the minister; finally and in a unique way under the species of the Eucharist. Hence, during Mass, the altar (where the Eucharist is celebrated) now becomes our primary focus. It is to the altar that we bow.
We also bow during the Apostles Creed when we say the words, “who was conceived by the Holy Spirit, born of the Virgin Mary” (or in the case of the Nicene Creed when we say, “and by the Holy Spirit was incarnate of the Virgin Mary, and became man).