The GY6vids guys sit down and go over the fundamentals of just what the heck is parallax adjustment, or adjustable objective (AO) on a rifle scope.
Sure, you know the standard two knobs (or tactical turrets) on a scope are for windage and elevation adjustment– but what about the third knob near the rear of the scope?
Its for adjusting parallax.
Why is it important? What’s it used for? Do you need it?
Watch the video, duh.
Source Article from http://www.guns.com/2015/09/01/the-basics-of-scope-parallax-adjustment-video/