An "active skill" is a Tier 4 skill found in any of the four main skill trees. Once you unlock it, you will press
during a mission to toss out a care package of sorts, and one of your teammates must interact with it to get the achievement. You can only deploy it once per mission, so make sure you throw it out when your guys are close by.
You can actually interact with your own care package but I'm not sure if this unlocks the achievement.