xmodmap

1. keycodes are generated by the hardware and vary by keyboard.
2. keysyms are logical names.  xmodmap lets you say which keysym a keycode will emit.  you can say that <keycode> will emit a when unshifted, A when shifted.
3. modifiers are another layer of indirection.  the keysyms Alt_L and Alt_R can generate modifier Alt.  The modifers include shift, lock, control, mod[1-5].
4. applications listen for these modifiers.  so bbkeys is described in terms of mod1, mod2, for example.