A responsive creative means that when someone adjusts the size of the window where the ad is being shown, the ad will adapt to fit. Your material must be delivered in HTML in order to be responsive.
Here's an example of a responsive creative: Fluid Skin. Try changing the size of the window where the creative is being shown, and you will see that it adapts.
Navigate to our ad specs to learn more about the requirements for each of our formats: www.adnami.io/specs