I suggest looking HERE as it has all you need to know about fusebox with PHP.
But in a nutshell, here's the absolute basics:
Create a main application page (for your app) with a case/switch structure.
nest further application pages (as above) inside each switch (as above), for each sub area/section of your app. untill you get down to the bottom level.
Thats why its called Fusebox (a box (application) full of fuses (sub apps))
track navigation variables like this: app.php?fuseaction=mainapp.subapp (this is what the switch structure looks at)
p.s. I have not read details on it for about 1 year (maybe more), and I read the docs for coldfusion. But as I say this is the basic idea.