Typically a disabled app is one that was pre-installed on your device by the manufacturer or carrier. The ability to disable those was a concession Google made in Android to allow people SOME control over what apps were visible in the app drawer. The disable button replaces the uninstall button that you see on downloaded apps.
Unfortunately, if it truly was a pre-installed app (if it was there when you first booted the phone this is the case), there isn't a whole lot you can do without rooting the device. There are other ramifications if you completely remove the app, it had been previously reported that removing pre-installed apps like that would cause any over the air updates to fail but I do not know if that is still the case.