All objects in our sceneries usually have a pre-programmed maximum altitude after that they will be discarded.
If the Altitude Cull option is disabled, this altitude will be ignored, and the objects will always display, provided they are inside the airport range and their own visibility range (which is instead controlled by the Anti-popup slider).
It's an option mostly useful in FSX and slower PCs, to keep the fps up and reduce the risk of OOMs, since the objects will be destroyed and memory reclaimed if the airplane is over the threshold altitude. But, just like the anti-popup option, it might add some stuttering so, as usual, there's no free lunch and, by gaining some fps and saving memory, you lose smoothness somewhat.
Under P3D4, it's probably best to keep it disabled.