The link below has a fairly simple explanation.
As with iron sights, there is one particular position for your eyes in relationship to the sight and the target that works (ie, proper sight alignment).
Cock you head over or move a handgun just 0.5" sideways, and you no longer have target and both sights aligned.
The same is true for a powered (magnified) scope. For each magnification the parallax adjustment is different. The higher the magnification, the more critical parallax adjustment becomes. And variable scopes will need new fine tuning once magnification is changed.
A 1x red dot sight (top models from Aimpoint, Eotech etc) is parallax free as the red dot is projected far out into the distance, basically putting dot and target in the same plane.
Therefore you can look through one of these scopes from all sorts of extreme angles, yet where the dot is, remains your point of impact. The dot also does not need to be centered in the scope at all.
Another different but related factor with powered optics is that a certain distance is required between the ocular and your dominant eye; this is the eye relief.
Here's the link: