Prostration can carry the symbolism of death–the death to self that comes before the candidate’s rebirth into priestly service. At some ordinations you will even see shrouds placed over the prostrate candidates.
Prostration is a rare sight. Outside of ordinations, the only other time a mass-going parishioner might see a prostration is during a Good Friday service, the day when we await the resurrection and “participate in [Christ’s] shock, and his descent into the depths of anguish.”
In the ordination, prostration also represents the candidate’s overwhelming humility before the call to priesthood. As Pope Benedict XVI recalls:
I shall never forget lying on the ground at the time of my own priestly and episcopal ordination. When I was ordained bishop, my intense feeling of inadequacy, incapacity, in the face of the greatness of the task was even stronger than at my priestly ordination. The fact that the praying Church was calling upon all the saints, that the prayer of the Church really was enveloping and embracing me, was a wonderful consolation.