Parallax in a scope makes it appear as if the target moves in relation to the reticle if you change head position (eye position) as the rifle remains steady.
The result is you can hold the crosshair in the right spot, but it could really be pointing an inch or more off that spot at 100 yards. This isn’t a big deal, but as distance increases, that one-inch difference expands to 2 inches at 200 yards, 3 inches at 300 yards, etc. For precision, long range shooting, a tiny big of parallax in a scope can result in significant misses.
This video clearly shows the parallax effect. Come along to see and learn more.