- 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 your threat on the unit specified is at least the threat level you choose. If you have no threat on that unit at all, it does not match.
Why you might use it
Checks your threat situation on a unit, from no threat risk up to tanking, secure. A damage dealer or healer uses it to drop threat when about to pull aggro (Feign Death, Fade, Soulshatter, or simply a weaker filler), and a tank uses it with NOT to Taunt when the mob is not on them. It is false when you have no threat at all on that unit. A level also matches when you are at a stronger one: tanking securely counts as tanking, and tanking counts as having at least as much threat as the tank. On Retail and WoW Forever the game can hide threat for some units (your target and nameplates are generally readable), and the condition is unavailable then. See also Threat Count.
Example
you have at least as much threat on your target as its tank
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.
Threat - The amount of threat you have with Unit. This condition will not be successful if you have no threat whatsoever (that is, you are not considered in combat with that unit). It is successful at the chosen level or any level below it in this list: tanking securely also counts as tanking, and tanking counts as having at least as much threat as the tank.
no threat risk - You are not in danger of taking threat away from whoever has threat.
higher threat than tank - You are in danger of pulling threat away from the current tank, and should reduce the amount of threat generated (by using threat-reducing abilities or ceasing DPS or Healing) immediately.
tanking, at risk - You are currently tanking Unit, however you are not at the top of the threat table, and thus are at risk of them switching targets to somebody else.
tanking, secure - You are currently tanking Unit and are not at risk of them switching targets at the moment.