defiance bar...
Are you talking about the break bar? (The blue bar under the enemy’s red hp bar)
If so, its easy to explain. Think of it like an “hp bar for crowd control efects”. When it runs out, the enemy gets stunned/dazed. Anything that would be considered CC (light or heavy) will chip away at it. The harder the CC (things like Knockdowns/Knockbacks) will do big chunks of it at once; while softer CC (Like Imob, Chill, Blind, etc) will chip away at it slowly.
In a way, you can pretend Knockbacks and such are like crit/power hitting an hp bar. And the imob/chill is like doing torment/poision. Any time the mob would be effected by the stun/condition, it hurts their break bar to allow them to ignore the effect.