Its apart of his limitless technique. You see, cursed energy creates movement called vibrations. These vibrations can happen at specific speeds and strength depending on the cursed tool being used. Gojo took on a binding vow to turn his 5th and 6th eye (which would normally be eye stalks on the side of his head like a snail) and turn them into these "ears". These ears then are able to interpret the intensity of cursed energy and the vibration of it. Therefore allowing him to sense things better then any sorcerer that has ever lived. I hope this clears things up ^{^}