Ossuaries are containers for storing skeletal remains. They can range from small boxes for individual remains to entire structures, such as the Paris catacombs. Many cities in medieval and early-modern Europe struggled to find space for new graves. Sometimes the bones from old graves would be removed and placed in ossuaries, making space for new graves. Some large ossuaries have elaborate decorations made of bones, such as garlands and mosaics.
There are many examples of ossuaries in medieval art, especially in my favourite subject for art, The Dance of Death, which depicts the dead dancing and playing music while leading people to their graves. Kings, peasants, young, and old all participated in the dance as a reminder that death can come to anyone and at any time. This site has a ton of examples of various dances of death from many periods.
Oh look! Here are some skellies getting up to have a dance now! Not to worry, they're not interested in claiming souls at the moment, just enjoying a nice tune.