Here's a link, so everyone can play along.
http://www.plcs.net/chapters/oneshot18.htm
The bits in question can be anything, but perhaps it's best to think of address 0000 as being driven by a Pushbutton, and address 1001 is an internal address which drives an output for Pilot light at 0500.
The upshot of the logic is that, when you push the pushbutton, address 1000 is ON only for one scan. But that causes the state of the light to toggle ON on the first push, OFF on the second push, ON on the third push, and so on.
Phil could perhaps have used 0500 everywhere instead of 1001, but not all brands of PLC allow you to reference output addresses in contacts (I don't think they make those kinds of PLCs anymore, but I think I remember hearing about them). Plus, it's a little easier to follow the code if you aren't worrying about whether the light is ON or not while you're evaluating the bits.
I wouldn't call rung 2 "interlock" logic - it's a classic toggle, probably the most commonly used. "Interlocking" is where you prevent an output from comming on unless some (safety) condition is met.