I'd always thought that it was a rating of both clarity and signal strength with a different rate for each, say 5x4 would mean 5 for transmission clarity and 4 for signal strength. or the other way around. But that's just an assumption.
Yep... signal strength and clarity! I always get confused which is which, but apparently this comes from back in the day. Not so much for aviation talk, but "Loud and Clear" is used a lot at workplaces/etc too like some have mentioned!