Parallax error is looking at something that is relative to something else and not approaching it squarely. Think of an old Simpson 260 meter with a mirrored scale. The needle stands about a quarter inch from the scale so you have to be squared up to get an accurate reading. If you are not square you will see the needle reflection in the mirror.
A scope's crosshairs are relative to the target. To see the error in a scope, get the rifle on bags at the range so you don't have to hold it. Now look through the scope and move your head slightly left to right and up and down. The crosshairs will move on the target. Now slowly adjust the AO while testing for parallax error again. It will disappear with adjustment. AO will also help with focus at different ranges.
