Parallax
An optical effect where the reticle and the target sit on different focal planes. Moving your eye behind the scope shifts the reticle relative to the target.
You see parallax most clearly at high magnification. Move your eye left and right behind the eyepiece. If the reticle slides on the target, the scope is not parallax-corrected for that distance.
Most precision scopes have a side-focus or adjustable objective knob marked with distances. Set it to the actual target range and parallax goes away. On a fixed-parallax scope, the manufacturer picks a distance and ships it.