- 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 whether the combat event you choose happened to the unit specified at a position in its recent history that fits the comparison with the count you set, where 1 is the latest event and 2 the one before it. Events older than the Combat History Memory setting are forgotten.
Why you might use it
Looks back through a unit's recent combat events by position, where 1 is the latest event and 2 the one before it; the action is what happened to that unit, so 'you were blocked' means you blocked an attack. The classic use is a Warrior lighting up Revenge after a block, dodge or parry, or Overpower after the target dodges. Only you, your pet and your target can be chosen in the editor, and events older than the Combat History Memory setting are forgotten. To look back by seconds instead, see Combat Action History Time; on Retail and WoW Forever an event the game hides makes that position unavailable.
Example
you were blocked at most 2 actions ago
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.
Action - The kind of combat action to look for (Hit, Miss, Parried, etc).
Operator - How to compare Combat Action History with the Count value. This comparison uses basic mathematical comparison operations.
Count - How far back in your combat history to look for Action (by count). A value of 1 means the last combat action, 2 means two combat actions ago, etc. Each combat action is treated separately. Any combat action more than the setting of Combat History Memory (seconds) in the primary Rotation Master configuration screen ago will not be available.