They're talking about determining the origin of a sound. They're saying if a sound originates from anywhere on the line of the cone shape it becomes impossible to decipher where it originated accurately. Humans actually have decent vertical echolocation, but just like dogs do when they're trying to vertically echolocate we can also just face the approximate sound origin and tilt our heads to improve echolocation accuracy because our brains use the difference in elevation between our left and right ears to pinpoint the sound's location.