The following is an explanation about the cord from the Sanctuary of Saint Philomena in Italy:
This pious practice, born spontaneously among the Saint?s devotees, was approved by the Congregation of Rites on September 15, 1883. Later, on April 4, 1884, Leo XIII enriched it with precious indulgences. It consists in carrying, tied around the body, a cord of wool, linen or cotton, colored white and red to indicate the virginity and martyrdom of Saint Philomena. The devotion is very widely practiced, especially outside Italy, to obtain spiritual and corporal graces. It has been made obligatory for those who carry the Cord to recite each day the following prayer:
Oh Saint Philomena, Virgin and Martyr, pray for us so that through your powerful intercession we may obtain that purity of spirit and heart that leads to the perfect love of God. Amen.
In the early Church virgins wore a cincture or cord as a sign and emblem of purity. That is why the cord has always been considered a symbol of chastity as well as mortification and humility. The wearing of a cord or cincture in honor of a saint is of very ancient origin, and we find the first mention of it in the life of St. Monica. In the Middle Ages cinctures were also worn by the faithful in honor of saints, though no confraternities were formally established, and the wearing of a cincture in honor of St. Michael was general throughout France. Later on, ecclesiastical authority set apart special formulae for the blessing of cinctures in honor of the Most Precious Blood, our Lady, St. Francis of Paul, and St. Philomena.
This extraordinary privilege of a cord is shared with the still other saints, such as, St. Francis of Assisi, St. Thomas Aquinas, and St. Joseph.
The devotion of the Cord of St. Philomena was adopted on account of the innumerable graces obtained through the intercession of the Hidden Saint of the Catacombs. This Cord was first distributed by the saintly Cure of Ars, himself. It was the illustrious Leo XIII who blessed and approved the Cord and attached many privileges and indulgences to its wearing.
Many records tell of the favor granted to those who wear the Cord. We read of striking cures; for instance, when wound about diseased limbs, the Cord has driven out the pain and healed them. There is also the story of the child who fell into a pool of water, but was preserved because of the Cord of St. Philomena around his waist. The Cord itself remained perfectly dry.
Rules For The Cord
The Cord approved by the Sacred Congregation in honor of St. Philomena ought to be made of linen, wool, or cotton of a white and red color, with two knots at one end to honor her double title of virgin and martyr.
It is worn under the dress as a girdle. No ceremony is required in conferring it, but it should be blessed beforehand and passed or sent by one person to another. When worn out, the new Cord also must be blessed. Children, however young, may receive the Cord.
In putting on the Cord, everyone ought to propose to himself constantly to honor St. Philomena, in order to merit her protection against all evils of soul and body, and also to obtain, through her intercession, perfect chastity, the spirit of faith so necessary in the unhappy times in which we live, and the grace of doing violence to oneself so as to lead a Christian life.
Those who wear the Cord are counseled to say every day the following prayer:
O Saint Philomena, virgin and martyr, pray for us that, through thy powerful intercession, we may obtain the purity of mind and heart which leads to the perfect love of God.
Leo XIII, by a rescript dated March 14, 1893, granted 100 days? indulgence to the faithful who wear the Cord of St. Philomena, and recite the foregoing prayer with devotion and contrition.
You can find much more about Saint Philomena from the official website of the Sanctuary of Saint Philomena: