- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks how much the unit specified resists the spell school you choose, as a percentage, compared with the percentage you set. It relies on MobInfo2 records, so a mob you have never fought does not match.
Why you might use it
Compares a unit's recorded resistance to one school of magic, so a caster can pick a better school: a Mage switches from Fireball to Frostbolt against fire-resistant mobs, or a Warlock leans on Shadow Bolt only when the target does not shrug off Shadow. The numbers come from the MobInfo2 addon, which learns them from your past fights with that kind of mob; they can be inaccurate, and the condition stays false until you have met the mob at least once. Without MobInfo2 installed it does nothing. On Retail and WoW Forever it is unavailable when the game hides the unit's name. See also Immune.
Example
your target's resistance to Fire is greater than 50%
Settings
What each setting in the condition editor means, from the game's own help.
Unit - The unit this condition uses as the target of comparison.
Spell School - The school of magic resistance to check.
Operator - How to compare Resistance with the Resistance value. This comparison uses basic mathematical comparison operations.
Resistance - The resistance of Unit to the specified school of magic as a percentage
This condition uses data from the MobInfo2 addon. As such it relies on previous interactions with the type of mob in question, which may be inaccurate. Additionally, until you encounter the mob at least once, this condition will always be false.