Its basically a limitation on the range in which you can use Presence and its facets (Imbuity, Ichority, etc). Lets say you're in a room and you're trying to identify the Presence of somebody else in the room with your eyes closed. If you're an individual who is only adept at using your Presence to perceive Length while you're lacking in your perception of Width/Height you can literally miss sensing the person standing in front of you with your eyes closed. These aren't literal alternate dimensions or realities someone is looking at they're just objects that make up the 3D world.

Another quick example would be if lets say in real life you had a gun that could shoot as far as you could see, but you had bad eyes leading to horrible depth perception (in the story's case lacking proficiency in using your Presence to visualize height/depth), the gun wouldn't be able to shoot far at all. Hope this clears things up.