Parallax happens when you, for example, view a clock from the side. Your view of the time (assuming it’s a clock with hands) will be slightly different than someone viewing the same clock head on.
Zeroparallax is the idea that you understand both how you relate to the clock and how someone else does so that you can account for the differences in perspective.
The more perspectives you have, and you gain them from experience, the deeper your understanding.