A scope can only have perfectly zero parallax at one distance. That's usually 100 or 150 yards for "centerfire" scopes, 50 to 75 yards for rimfire or shotgun scopes. All it means is that the reticle and the image within the scope at which you are looking are at the same point...the crosshairs are plastered right on the aiming point, so no matter how you move your head, they are perfectly superimposed. If you are looking at something which is closer or further than what that scope is adjusted for, the image will be focussed at a slightly different distance from your eye, but that pesky reticle will still be in its fixed position, which means that if you aren't perfectly centered behind the scope you can peek "around" the reticle at the image, causing the reticle to apparently move on the image. That is parallax-induced error, but the error is really yours...if you are perfectly centered, the parallax will still exist, but there will be no error and your accuracy is unaffected. You introduce the error by looking around the reticle at the image.
It's more noticeable with higher magnification, but it's still there with a 2-7, just not as noticeable. And the amount of this error is more limited at short distances. If the scope is set for 100 yards and your target is at 20, the maximum parallax error is still not all that large, and the only way to get that maximum error is by having your eye way off center, so that you are only seeing a portion of the field of view. But if your target is at 1000 yards, and your scope is set for 100, parallax error can be much larger, and much more noticeable.
Sure, you can buy a scope with a parallax adjustment feature. Technically, what that means is that you can dial out parallax for any range, removing that potential error. But bear in mind that the distances marked on the adjustment dial or ring are far from accurate, and that the depth of focus is so large that it takes a trained eye to know when you are actually properly focussed; the odds are that the parallax adjustment feature will probably just mean that instead of only
some of your shots being subject to parallax...
all or at least
most of them will be.
Learn proper form, holding your head and eye consistently behind the center of the scope, and you will be much further along the road to practical field accuracy.
Edit: Sorry, I type too slow...grelmar beat me to it, and in way fewer words to boot!
