It's the same ability that emperor's children have, basically the new "always strike first" of 8th edition 40k. Basically what it means is your succubus will always count as having charged in terms of combat activation, so she can always activate her attacks before an enemy unit that has not charged this turn. However, if say your succubus is charged by an enemy unit, that unit also strikes first because it charged, so then you and your opponent alternate activating units that have charged or have a similar strike first ability.
Basically what it all comes down to is in scenarios where your succubus is in an ongoing combat, she'll always strike first. In scenarios where she is charged, your opponent will get one activation, then your succubus gets to go. It's basically like using the interrupt stratagem from the rulebook without spending any cps.
The Night is Dark and Full of Terrors...