It's what happens when you dont look stright into the analog meter movement when taking a reading.
In other words ... you look at an angle and see a distorted view and your reading is wrong.
Anyone who grew up in this trade with analog meters knows what paralax is. That is .... if your an "ol fart"!
Anyone who dont know what paralax is is either a newbie to the industry or perhaps just had a very sheltered life ....