- 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 of its main power, such as rage, energy or mana, the unit specified has, compared with the number you set. A negative number compares how much power the unit is missing from its maximum instead.
Why you might use it
Compare a unit's primary power as a raw number: rage, energy, focus, runic power and so on (for mana, Mana is clearer). A Warrior dumps spare rage into Heroic Strike above 60 rage, a Rogue pools energy before a finisher, and a Death Knight spends runic power on Death Coil; a negative value compares missing power. Not available on Retail or Forever, where the game hides these values from addons. See also Power Percentage and Points.
Example
your power is greater than or equal to 60
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.
Operator - How to compare Power with the Power value. This comparison uses basic mathematical comparison operations.
Power - The raw power value of Unit. Power is a statistic that is class (and sometimes spec or form) specific (eg. Warriors have Rage). If this number is negative, it means the power deficit (from max power).