Parallax is seen when you move your head around while looking through a scope and the crosshairs seem to move on the target. If your scope is set to be free of parallax (at a given range) when you move your head the crosshairs will appear to be fixed, or welded to the target.
Some scopes come set to be parallax free at 50 yards, others at 100 yards, while some have adjustable objectives that allow you to dial out the parallax at different ranges.
What type of scope you need depends on how youll be using it. I like adjustable objectives for target shooting. For the type of hunting I do I like my hunting scopes to be factory set for parallax at the yardage I expect to get my most of my shots. Afterall, its not too often that youll have a squirrel, deer, or coyote stand around and wait for you to get your parallax adjusted