What the hell is that mean?
Quoting the Wikipedia page for “GUI widget”:
In the context of an application, a widget may be enabled or disabled at a given point in time. An enabled widget has the capacity to respond to events, such as keystrokes or mouse actions. A widget that cannot respond to such events is considered disabled. The appearance of disabled widget is typically different from an enabled widget; the disabled widget may be drawn in a lighter color, or may be visually obscured in some way. See the image to the right for an example.
This concept has been around for a couple of decades and can be found in most GUI frameworks.
what is the difference between enable state/clickable state and disabled state/ not clickable state?
In Android, a widget that is not clickable will not respond to click events. A disabled widget not only is not clickable, but it also visually indicates that it is disabled.
what do you mean by: “..since it makes the Button visually “disabled”? how does it changes it visually?
What makes a Button look and respond like a Button is its background, which is a StateListDrawable. There is a specific image used for the disabled state.