Value is defined on a per sctrl (state controller like type = velset) basis. In things like changestate it refers to the state you'll go to. In something like changeanim it refers to the anim you change to. Refer to your docs\sctrls.html for info one what it does each time.

Physics tells mugen what sort of velocity management to apply to the state. C and S are friction, A is apply gravity. N doesn't do anything

Velset does what it says. Sets velocity.

Ctrl = control. 1 means your keys work, 0 means they don't.

X... Like, math? X is horizontal motion or axis. Yeah, you need some tutorials at this point. And some more messing round than just copy paste code.