When you're actively afraid, you know it. You can feel the fear pressing against you. Maybe you work through it and conquer it this time, or maybe you don't. But if you succeed, you know you worked through the fear. And you know if you fail, the fear beat you.
This is the type of fear you feel when you're about to do public speaking, make a cold call, ask for a raise at work, or do a difficult ski run.
And maybe sometimes you back down from whatever it is you were afraid of. It happens, and it's not the end of the world when it does. Fear winds up beating most people at least sometimes, in some domain.
Passive fear, on the other hand, is far more insidious. It's what Pressfield called "The Resistance" in The War of Art. It's a form of fear, but not one that shows itself. It's sneaky. You find the need to do some lower importance activity instead of doing your work.
Excuses abound, the important things don't get done.
Active fear is fine, even on the occasions you lose to it. You know the enemy, and you know it beat you this time. Fine. No problem. Get stronger, take action next time.
Passive fear creeps into the back of your psychology, making you avoid an area where you'd feel at risk of putting yourself out there. But usually, you don't realize it's even fear working against you. Just... you don't have enough time, you're too busy, you're too tired, there's too much to do, you'll get to it tomorrow...
...or worst of all, you're not even thinking about the biggest goal you've got right now.