- 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 it should take the unit specified to reach the health you set, compared with the number of seconds you set, estimated from recent damage, healing or both as Mode says. A negative health number means that much missing health instead.
Why you might use it
Estimates how many seconds until a unit reaches a given health value, from the recent rate of damage and healing on it. Its main use is not wasting long effects on a mob that is about to die: skip Rend, Corruption or a long cooldown when the target will reach 0 health within a few seconds. A negative Health means a deficit from maximum health, and Mode picks whether damage, heals or both feed the estimate. It does not exist on Retail or WoW Forever, where health and the combat log are hidden from addons. See also Time Until Health Percentage and Health.
Example
time until your target's health reaches 0 is less than 6 seconds with Damage and Heals
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.
Health - The target health the rule is waiting for Unit to get to. If this number is negative, it means the health deficit (from max health).
Operator - How to compare Time Until Health with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - How many seconds (estimated) until Unit reaches the target health.
Mode - How the burn rate should be calculated, which is used to estimate the time to target health.
Damage and Heals - Both damage and heals on Unit will be factored into the 'burn rate' (essentially mitigating damage with heals for a realistic burn rate)
Damage Only - Only damage on Unit will be factored into the 'burn rate', which will cause some spikes in time-to-target if they are healed.
Heals Only - Only heals on Unit will be factored into the 'burn rate', essentially pretending they are not being hit at all.