Spell damage resistance is a special ability of several creatures. It's characteristic of Tower's Stone and Iron Golems as well as neutral Gold Golems and Diamond Golems. It reduces damage from spells by a specific percentage which depends on the creature type. However, it does not prevent spells from working as the manual claims.
Creatures with spell damage resistance:
Stone Golem - resists 50% of the damage from spells
Iron Golem - resists 75% of the damage from spells
Gold Golem - resists 85% of the damage from spells
Diamond Golem - resists 95% of the damage from spells
Firebird - originally fully immune to Fire Magic spells, in HotA 
resists 50% of the damage from Fire spells
Steel Golem - resists 80% of the damage from spells