"Magic being" is a tag found on units and commanders that are highly magical or otherworldly. These could be walking statues (Sentinel), elementals (Fire Elemental), holy beings (Harbinger), eldritch abominations (Illithid), etc.
This is generally a bad tag to have, even though many powerful units have it, since it creates logistical headaches and special vulnerabilities:
However, units and commanders which have both the Never Heals and the Magic Being tags will be able to recover their HP in a lab.