Sure, I'll give it a try. I don't have a lot of formal training with this so I'll do my best to describe how it works.
So basically, most of us think of relays as switching from OFF to ON when you apply power to the appropriate lead of the little miniature electromagnetic gizmo inside them that does the actual work of switching. And a lot of relays work that way. But some switch from ON to ON, they're just switching which lead is getting +12v. You can use that kind of relay as OFF/ON by not connecting (and insulating, please!) one of the ON leads, if that makes any sense. But you can also connect something (like a starter relay) to one of the ON leads, the one that is selected if you apply +12v to the control circuitry, and something else (like a horn relay) to the other ON lead, the one that is selected if you don't apply +12v.
What makes this tricky is that if you stop there, the horn would just honk endlessly while the truck is parked. There are a few ways out of that situation, one of them being to make the horn relay ignition hot (only gets +12v when ignition is on) instead of always hot. This does mean you won't be able to honk if the truck is turned off, but probably you weren't planning to do that anyway. It also means the factory alarm won't be able to honk if the truck is turned off, though.
So when the truck is at rest, nothing happening, the horn circuit is connected but is not being energized because there is no +12v to the horn relay.
When someone tries to start the truck without hitting the kill switch, the horn circuit is energized. Meep meep!
When someone tries to start the truck and hits the kill switch, the relay switches power to the starter relay, and starts the truck.
To say the obvious, kill switches do add unnecessary circuitry that can fail and strand you. If you install one, make sure you understand how to undo it if the relay fails far from home.