Hold the glass an inch over the mark and move it around slightly in different directions.Put an inch or so of water in a clear drinking glass.Put a small mark from a pencil on a table.Here's a simpler example of how parallax works: This will also happen if the target is set at a much further distance, say 300 yards. The scope's 'eye' is not focused at the proper distance.
If you take one of these scopes that are set at 100 yards and put it in a vise on high power with the crosshairs centered on the bullseye of a target about 25 yards out and move your head slightly off-center to the left and right or up or down, you'll see the crosshairs move off the bullseye, even though the scope has not moved. Most rifle scopes in a medium-high variable power range, say 10X or slightly higher, which often don't have external parallax adjustments, are parallax corrected at either 100 or 150 yards. One way to help explain it is to picture the following:
Parallax is hard for most people to understand.
You need to adjust for parallax to ensure your reticle will be accurate at any viewing angle. If you notice your reticle is moving off target while changing your viewing angle, that's parallax. A simple definition of Parallax: a noticeable shift in reticle placement while looking through your scope at different angles
