Read about this subject here. TL;DR: The cleaner the headphone is between frequencies, especially in the high frequency region, the more definition it can have, and as a result, you will notice more "details." These descriptions correlate with the idea of auditory masking.

Among audiophile reviews and headphone frequency response graphs, what correlates with the idea of "more detail" is usually the region above 10kHz.

The region above 10 kHz is usually reduced in headphones as it is a region that we hear at high volume, but some headphones made for audiophiles have a lot of activity above that region so that the details in that range can be heard more easily.