- 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 much time is left on your boss mod's timer for the boss ability whose spell ID you enter, compared with the number of seconds you set. It does not match while no timer for that ability is running.
Why you might use it
Compares the time left on your boss mod's timer for one boss ability, identified by the ability's spell ID. Use it to get ready for a mechanic: a defensive or shield shortly before a big hit, or no long cast started just before a knockback. It is false while no timer for that ability is running, the timers are the boss mod's predictions and can shift, and it is unavailable without DBM or BigWigs. It is offered on the Classic editions only, not on Retail or WoW Forever.
Example
the boss-mod timer for Wrath of Ragnaros is less than 3 seconds
Settings
What each setting in the condition editor means, from the game's own help.
Compares the time left on your boss mod's timer for one boss ability, identified by the ability's spell ID; while timers run, the editor lists them with their IDs. Timers are the boss mod's predictions and can shift, pause or be canceled. When no timer for that ability is running the condition is false, so 'is less than 5 seconds' is only true while a timer is actually counting down. Bars without a spell ID, such as pull or respawn timers, are not tracked. Unavailable when no boss mod is enabled or loaded.
Operator - How to compare Boss Mod Timer with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - Seconds until the boss mod expects the ability. A paused timer keeps its remaining time.