- 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 in the current channel of the unit specified, compared with the number of seconds you set. It does not match when the unit is not channeling, whatever the comparison.
Why you might use it
Compares the seconds left in a unit's current channel. On yourself it lets the next spell light up just as a channel such as Arcane Missiles, Drain Life or Penance is ending, so you can queue it without clipping; on an enemy it shows how long you have left to interrupt. It is false when the unit is not channeling, whatever the operator, and on Retail and WoW Forever it is unavailable when the game hides another unit's channel timing. See also Channel Progress.
Example
the time remaining on your channel is less than 0.5 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 Channel Time Remaining with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - The number of seconds remaining in the current spell being channeled by Unit. If the Unit is not currently channeling, this condition will not be successful (regardless of the Operator used.)