- 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 damage is waiting in your Stagger pool, or that damage as a percent of your maximum health, compared with the number you set.
Why you might use it
For Brewmaster Monks: use Purifying Brew, or hold it, by how much Stagger you carry. Tick "As a percent of maximum health" to compare against your health instead of an amount. Mists of Pandaria, Retail and WoW Forever only. On Retail and Forever the game can hide the pool from addons in combat and always hides maximum health, so the percent form is unavailable there and the amount is unavailable whenever it is hidden; neither is ever guessed.
Example
your Stagger as a percent of your maximum health is greater than 20 percent
Settings
What each setting in the condition editor means, from the game's own help.
The damage waiting in your Brewmaster Monk's Stagger pool, or, with the box ticked, that damage as a percent of your maximum health. Mists of Pandaria, Retail and Forever only. On Retail and Forever the game can hide the pool from addons in combat, and always hides maximum health, so the percent form is unavailable there and the amount is unavailable whenever the game hides it; neither is ever guessed.
Operator - How to compare Stagger with the Value value. This comparison uses basic mathematical comparison operations.
Value - The damage waiting in your Stagger pool, or the same as a percent of your maximum health when the box is ticked.