Parallax is the apparent movement of the cross hairs on the target if your eye moves slightly behind the scope.
This is caused by the plane of critical focus within the scope not being exactly on the same plane as the reticle.
Less of a problem in fixed power scopes.
Can be corrected in a scope with a parallax adjustment. Basically a knob to bring the plane of focus in line with the reticle.
Several good articles are on line.
It's smaller on the outside.