From the homily of Pope Francis at his first Chrism Mass as Bishop of Rome:

The sacred robes of the High Priest are rich in symbolism. One such symbol is that the names of the children of Israel were engraved on the onyx stones mounted on the shoulder-pieces of the ephod, the ancestor of our present-day chasuble: six on the stone of the right shoulder- piece and six on that of the left (cf. Ex 28:6-14). The names of the twelve tribes of Israel were also engraved on the breastplate (cf. Es 28:21). This means that the priest celebrates by carrying on his shoulders the people entrusted to his care and bearing their names written in his heart. When we put on our simple chasuble, it might well make us feel, upon our shoulders and in our hearts, the burdens and the faces of our faithful people, our saints and martyrs.