Parallax is a word you’ll inevitably read and hear before to long when it comes to optics on firearms.
Certain optics have a parallax adjustment tool or knob. Certain reddot, reflex, and holographic weapon sights are advertised as being ‘parallax free’ or have so very little it matters negligibly. Others have a fixed parallax distance that covers a practical range for the optic.
But what is it? It’s a distortion caused by light passing through multiple lens combined with needing a proper position for the shooters eye behind the optic to get a proper sight picture. What you see is not always exactly where you are aiming.