It sounds like you're charging your iPad by connecting it to a USB port on your MacBook Pro.
If the message "Not Charging" appears in the status bar next to the battery icon, see this article for more information about charging your iPad:
One quick fix is to charge your iPad using the USB Power Adapter provided in the box.
If you're curious about how your MacBook Pro powers USB peripherals, you might find this article useful:
Have a good weekend,