What is this?
This statue is a "reliquary": a piece of art that holds a relic. In this case, the small glass tube holds a piece of an arm bone supposedly from the Christian St. Macarius of Egypt. Christian relics are the remains of holy people or objects, most commonly the body parts of saints or objects of Jesus', such as the crown of thorns. Relics have always had a very important role in Christianity, and they continue to be important today. Objects such as the crown of thorns or Mary's shroud are seen as inherently holy because of their direct connection to the foundation figures of Christianity.
Originally, relics were a way to celebrate the sacrifices martyrs made for their faith, but eventually, after Christianity was legalized, it expanded to include saints and holy people who were not killed in the name of God. Against theological doctrine, zealous believers thought that the bodies of the saints held their holiness even after death, leading to body part veneration. Over time, the tradition slowly morphed. Bodies were moved from simple graves to special tombs or churches so pilgrims could find particular saints easier. The bodies were also often run over with water, oil, or fabric to make "secondary" relics. This later shifted again, and bodies were stolen and separated into fragments and taken to legitimize churches and leaders.
In 1204, Crusaders sacked Constantinople on their way to the Holy Land. Until then, the city housed the majority of all Christian relics in exquisitely made reliquaries. This destabilized the core of the Byzantine empire and drove the leaders to sell their massive stash of relics to try to keep the government afloat. These relics were dispersed throughout Europe throughout the Crusades, giving them increased value as a status symbol for individuals rather than purely for rulers or religious institutions.