Adaptive EQ
In addition to ANC, the new AirPods Pro also have an advanced audio feature called Adaptive EQ. Apple says it’s a proprietary audio feature that “automatically tunes music to the shape of your ear for a rich, consistent listening experience.”
More specifically, it appears that Adaptive EQ will tune the lower and midrange frequencies of whatever is being played specifically based on the shape of your ear so that you end up with a consistent and custom listening experience. Regular AirPods, of course, don’t have this feature.