If you are unsure what parallax means on gun optics then you are right where I used to be. I am no expert but I now understand what this is and why it is so important. 

Check out the reflex red dot tube style sight from 2nd Amen Here.

In gun optics, parallax refers to the apparent shift in the position of an object viewed through the sight when the position of the viewer's eye changes. This can cause inaccuracies when aiming, as the point of impact may not align with the point of aim.

To eliminate this issue, many gun optics are designed with parallax correction, which adjusts the sight so that the point of aim remains consistent regardless of the position of the viewer's eye. This can significantly improve the accuracy of the shot.

Parallax correction is typically adjustable, allowing the shooter to fine-tune the sight for different distances. Some gun optics also have a fixed parallax setting, which is optimized for a specific distance and reduces the need for adjustment.

Overall, the use of parallax correction in gun optics can greatly improve the accuracy and precision of shooting, making it an important feature to consider when choosing an optic for your firearm.