[Edi]

Micah is correct. Units with morale 30 are not affected by spells like panic, terror or other similar morale reduction attacks. I'm not sure if they can still be affected by the fear ability.

Units with morale 50 are mindless and will be destroyed if the army routs and there is no commander to lead them on the field.

Interestingly, a commander with the Valor ability has increased morale and in that case the value can run over 30, though the display counter stops at 30. I know this because I once had a commander with Valor and he had morale 28 or so with the Battle Fright affliction. Would be nice to run a test where a commander with Valor was held in the HoF for a long time and see if he eventually turns mindless, though that would take a few hundred turns, I think.