A Muo-Ping was a sacred glass jar used to contain souls, in order to prevent them from travelling to their appropriate afterlife, or the ether. While they were mystical, they could be broken like any ordinary glass jar as what was sacred, was what was inside (soul).
One was used to contain Angel's soul after it was extracted by the shaman Wo-Pang. However it was then stolen by Jasmine (while she was inhabiting Cordelia Chase's body) from the Hyperion Hotel safe. Willow Rosenberg used the Delothrian's Arrow (in reality, a silver ball) to smash the jar in order to release Angel's soul into the ether, so it could be restored.