While I do not use that addon, it is probably telling you to use an execute ability to finish off an opponent. I use FTC and it has a target 25% health warning to help let people know to use an execute ability.
"execute" are ability with additional effect against low healt (25% or less) target... a few example of such abilities are "mage wrath" from the sorcerer's storm calling line or "reverse slash" from the 2handed weapon line
Dual Wield has the whirlewind aoe execute, and Nightblades have the ability assassin's blade.
I don't know of anyother ones, heck I didn't even know about the mage wrath ability. I have used the other three as my character is a nightblade and I have skilled into both 2hand and dual wield.
Edit: The bonus effect for the assassin's blade skill and reverse slash is a 300% damage bonus against low health targets. I'm pretty sure whirlewind just says it increases damage the lower health a target is. Can't check right now as I am at work.