A matrice is a mold onto which an impression is embedded of a particular design. These images were used during his papacy for the production of papal medals.
The obverse side is an image of Pope Clement XIII.
The reverse side is an image of Jesus washing the feet of a disciple from John’s Gospel, Ch. 13: 2-15.
2 When he had finished washing their feet, he put on his clothes and returned to his place. “Do you understand what I have done for you?” he asked them. 13 “You call me ‘Teacher’ and ‘Lord,’ and rightly so, for that is what I am. 14 Now that I, your Lord and Teacher, have washed your feet, you also should wash one another’s feet. 15 I have set you an example that you should do as I have done for you.
Pope Clement XIII chose this image for the matrice attesting to the importance of Jesus’ command to do what he has done.