it works like this : you apply an elemental status on a target (wont work if said target has a shield, or if X boss isn't in an "exhausted" state) with a primer attack (ranger melee, flamethrower, acit darts - spit, frost glaives, whatever else)
When said element is applied you can detonate it with a skill for a damage bonus. The trick is that it can only be detonated once, then you gotta re apply a primer and so on. Depending of the element you get various effects. The only subtlety about this is if you play interceptor, since he has skills that detonate primers in order for you to get damage auras. So if you get a fire aura you'll do fire dots damage, etc
Its not too complicated. also if you affect a large group of enemies with a primer and detonate them all at once with a skill, you'll get your ultimate skill much more quickly since you do that much more damage