Where does the number come from? - As far as I know, every single commercial plasma display ever made uses 8 or more subfields to make up one frame of video/film. And when refreshing at 60Hz this equates to 480 subfields per second.
8 subfields per frame x 60 frames per second = 480 subfields per second = 480Hz
If it is not new to Plasma then why use it? - Marketing against 120Hz for LCD
Is it important to motion just like 120Hz? - No
Then why even use this number in marketing? - Good question! I suspect because it is bigger than 120 and has "Hz" at the end of it 
Why is Plasma so good with motion then? - Plasma has a shorter duty cycle than LCD as well as a shorter display time than even 120Hz LCD