Equalizing... You adjust the "presence" or volume of each frequency range to come up with your desired sound profile.

So, imagine for example that you have some headphones that have too much harshness in the highs for your taste oraybe there's a peak in a certain frequency range that you want to get rid of or maybe you switched to sheepskin earpads and now you get too much bass... Well, essentially you can compensate by adjusting those frequencies with an equalizer to get the sound profile that you prefer...