The Tomb of the Holy Innocents is under the Basilica of the Nativity of Christ in Bethlehem. The relic is a piece of their tomb. They were martyred in the 1st Century and their feast day is December 28.
The Holy Innocents are the male infants of the town of Bethlehem, the birthplace of Jesus, whose deaths were recorded in Matthew 2:1-18. The infants were put to death by the soldiers of King Herod the Great (r. 37-4 B.C.) in order to prevent the fulfillment of the ancient prophecy that the King of the Jews would be born in the little town. The exact number of the slain is unknown, but it can be deduced that the population of Bethlehem during that period was not large. An angel warned Joseph of the impending danger and thus the Holy Family escaped into Egypt. The fourth-century poet Prudentius called the infants the flores martyrum, "flowers of martyrdom." According to legend, one of Herod's own sons, who was with a wet nurse in Bethlehem, was also slain.1
1 Bunson, Matthew E. Our Sunday Visitor's Encyclopedia of Saints. 2nd ed., Our Sunday Visitor Inc.,u.s., 2014.