- 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 far the unit specified is through its current cast, from 0% at the start to 100% at completion, compared with the percentage you set. It does not match when the unit is not casting, whatever the comparison.
Why you might use it
Compares how far through its current cast a unit is, from 0% at the start to 100% at completion, following haste and pushback; Cast Time Remaining gives the same in seconds. Use it to interrupt late in an enemy cast, for example at 70% so a fake cast in PvP does not waste your Kick, or to time your next action around your own cast. 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.
Example
your target's cast progress is greater than or equal to 70%
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 Progress with the Progress value. This comparison uses basic mathematical comparison operations.
Progress - How much of the current cast of BLIZ_YELLOWUnit is done, from 0% when it starts to 100% when it completes. It follows haste and pushback, because it is measured against the cast's own start and end. If the unit is not casting, this condition is not successful, whatever the operator; if the cast's times cannot be read, it is unavailable, including under NOT.