Zeroparallax means zero error. It’s a state of being where you know how you relate so that you understand, what you are relating to.
Using the zeroparallax position as a reference, you can calibrate for changes in the relationship so that even if you are “unaligned” you can account for this, or calibrate, and continue to understand, without error, what you are relating too.
Looking at a clock or dial gauge from the side, you can account for your viewing angle and thus make an educated guess as to what the dial or clock would show if you looked at it from straight on.
Knowing that a clock is 5 minutes fast you can account for this.
Knowing that a friend just had a bad day at work you can account for their behaviour…