"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:
Magic beings require a commander with magic-being leadership. For instance, a Cave Captain cannot lead Sentinels.
During combat, if an army has no commanders with magic leadership that are not routing, all of its mindless magic beings will stop moving and dissolve; magic beings with minds will rout.
Certain weapons do extra damage to magic beings (most prominently Moon Blade)
Certain spells only affect magic beings (such as Opposition and Arcane Domination)