- 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 many seconds are left before the current cast of the unit specified completes, compared with the number of seconds you set. It does not match when the unit is not casting, whatever the comparison.
Why you might use it
Compares the seconds left before a unit's current cast completes. Use it to interrupt an enemy's dangerous cast just before it lands, or to put up a shield or defensive right before a big spell hits; on yourself it helps line up the next action as your cast finishes. It is false when the unit is not casting, whatever the operator, and on Retail and WoW Forever it is unavailable when the game hides another unit's cast timing. See also Cast Progress.
Example
the time remaining on your target's cast is less than 1.5 seconds
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 Cast Time Remaining with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - The number of seconds before the current spell being cast by Unit is complete. If the Unit is not currently casting, this condition will not be successful (regardless of the Operator used.)