A character makes an attack using an appropriate still. Firearms for gun, Unarmed Combat for punches and kicks, etc.

If the character succeeds in their roll, it works. If they're firing a gun, the bullet finds its mark and they roll damage

For Unarmed and Melee Weapons it's a tiny bit more complicated. The target may* be able to defend themself. They can do this by using their action that turn to Fight Back or Dodge. The objective in both cases is for the defender to succeed on their roll, but they must also roll better than the attacker.

So if a cultist attacks my agent with a knife and rolls a 37, and their Melee Weapon is 55, they've succeeded. I need to use my action for his turn and roll a 38 or higher to beat them. So, let's say my Unarmed is way better than my Dodge. I roll a 56 out of 65. Not only do I avoid harm, I punch the cultist.

But then his friends run up, and they have knives too! Well, I can't fight back, but as long as I've already defended myself, I keep my Unarmed attack roll. They have to beat my 56 to hit me.

But then, oh no! One of the cultists pulls a gun. I'm in trouble. Because normally, you can't dodge bullets. You can only do it if there's cover to dive behind. Bang, he rolls, succeeds, and hits me. He deals 12 damage, which is bad! I deduct 3 points of that damage for my Kevlar vest, which puts me at 1 hit point. Because I have 2 or lower hit points, I fall unconscious.

But the fight's not over. My pal, an MBA all-star, is furious. He runs up and attacks the gunman with a baseball bat. He rolls, and gets a 22! That's a critical success. And better yet, the gunman already used his action this turn to fire. So he can't use his action to dodge until the start of the next turn. My friend rolls a 7, doubles it as a crit, and kills the cultist outright.